Inferential statistics I ─ Hypothesis testing in the basic form of conditional probability / Bayes' rule in R
PDF- Version of this tutorial:
Introduction ─ Structure of the series
Welcome to our first tutorial within our editorial collection “Stat-o-Sphere”. You have just entered the absolute beginner’s sphere, so we hope your stay will not come with any turbulent inconveniences and we wish you a pleasant and insightful trip through our tutorial. As this is our first editorial within this collection, we decided to give a brief overview over two concepts that we believe are most important for understanding scientific reasoning as such and that are also often heavily misunderstood ─ namely: the concept of hypothesis testing and statistical modelling. Knowing at least these two concepts in detail is especially for those of you important that seek to get a stable heuristic overview over what statistical inference is all about, without digging into every possible method.
As mentioned in the introduction of our tutorial collection, we are trying to provide you with slow-paced tutorials that potentially entail the conceptual / intuitive, the mathematical and the computerscientific perspective (programming) on statistical methods. In this tutorial series, we will provide an introduction into inferential statistics on all of the above ‘levels of abstraction’ at one place. Not every area might be of your immediate interest. This is also the reason why we established a short summary at the end of every chapter that can also be used as a chapter overview for the impatient reader. In the future, we will also add chapters going through the statistics of open data papers, which will provide you with a wider range of examples with different levels of complexity and further insight to the application of statistical methods in the wild, so to speak. Corresponding to a modular educational expansion of actual scientific work, published via our student journal, we will also add condensed chapters that will focus on aspects that are not only important for the application, but mostly for the interpretation of the results ─ e.g. within review processes.
This tutorial is therefore structured more or less hierarchically, moving from the intuition to the mathematics and eventually to the code in R, using functions such as lm() for linear models ─ all coming together in the interpretation of the output results (most important for review). Note that the first part of the series introduces R only to do calculations that can be done by hand or with a calculator as well! So in case you are scared to get into programming languages, fear no more. We are trying to introduce R as what it essentially is: a very sophisticated calculator.
You may have prior knowledge and find some of the math rather boring at the beginning. As the following tutorials are also introducing coding as such, we chose to start from a “absolute” beginner’s level. It also comes with the advantage that this tutorial can theoretically also be understood and mastered by scholars to some degree (we may test for that in the future).
We also want to give those a chance to get a full recap of the mathematics that didn’t start studying soon after school, or that are not that much incorporated in the matter for any other reason ─ especially those concerning a lack of interest or aversion. In the end, statistics paves the path of every medical inference nowadays and it does so for very good and even intuitive reasons, as we will see.
Statistical modelling will be the core of our first series of tutorials on inferential statistics, discussed on the basic example of a linear regression model (eventually moving to other methods in the future, such as linear mixed effects models). Hypothesis testing in the mathematical sense will be important especially when evaluating the results of our linear regression, i.e., the full output that we have obtained using the programming language R (especially when discussing the p-value, obtained via a t-test as a specific form of hypothesis testing on linear regression models).
However, hypothesis testing in its conceptual and mathematical sense is what statistical modelling is for in the first place, so we will start our journey into the Stat-o-sphere by going through its steps. This will give us a clear view on what a p-value actually is in terms of probability theory as well as conceptually, and will show us the rather banal difference between the frequentist and Bayesian interpretation of conditional probability. There is also a vast variety of mathematical algorithms, called “tests”, that all result and reflect on p-values of a model, so we decided that it is best to not start with a specific test for a specific type of model, but with explaining the concept of hypothesis testing itself and how the p(robability)-value represents itself not as any, but as specific conditionalprobability.
We believe that discussing conditional probability as a start is the most economic approach to statistics, as it provides a conceptually consistent overview over a vast variety of methods involved in statistical reasoning in general, apart from the p-value (to name a few: positive predictive value, even thermodynamics, information theory (Shannon), AIC, BIC, computational neuroscience (“Bayesian Brain Hypothesis”) etc.). Another reason is that hypothesis testing understood as conditional probability is actually really simple and especially surprisingly intuitive and doesn`t need any mathematical background at all to be understood.
Surprisingly, it appears as if most tutorials leave out the topic of conditional probability in the first place and jump right into using concepts such as variance, t-value, confidence intervals etc., so our approach will hopefully close an important gap for those seeking further insight and a clear intuition on each of the concepts by themselves and in relation to each other (which also involves a linear model, which we will go through first too, before we get to a commonly used t-test etc.). In other words: the p-value is a conceptual composition, which however still begins with conditional probability / Bayes’ rule.
If all of this still sounds a lot: The webcomic above shows that hypothesis testing involves nothing you don’t know or wouldn’t do already and we hope that our tutorial will leave you as surprised as we are, when realizing how easy the steps of testing a hypothesis can be represented by mathematical terms ─ without much of an effort and without losing any of its conceptual intuition and magic behind it.
1 Hypothesis testing
In general, every scientific study, every experiment, every process of ‘testing’ involves going through the following three steps in some way or another (even in descriptive statistics we explore data in relation to what we expect, the full argumentation is just not represented via mathematics):
formulating a (prior) hypothesis ─ that “what-ever something” is the case
gathering (new) data that is related to our hypothesis (events where the hypothesis holds)
evaluating the results (testing the hypothesis) and adjusting the prior hypothesis in order to better predict new data (updating the hypothesis)
So far this may not reflect on all the formulas and processes that pre-informed readers may expect, when performing statistical analysis. Nevertheless, it makes up the core of every statistical analysis in some way or another. Let’s go through them in detail:
1.1 The prior hypothesis
In the beginning of every scientific evaluation there is a claim, i.e., a prior hypothesis. A prior hypothesis can be understood as a belief about the world, regardless any present or future experiences, or in other words: before an experience was made that could prove or disprove the hypothesis (the prior hypothesis can also be looked at as the sumof all pastexperiences on a hypothesis). An “experience” in statistics is often called an event. In general, an “experience” or an event is termed data in statistics, which represents events in the form of the outcome of measurements of any kind ─ measurements that have not yet been made to this point of inquiry.
1.2 Gathering evident data
The next step in any scientific investigation is a process of gathering experiences, observing events ─ gathering data. This can be done in various ways. However, there are certain constraints to what can be considered data, which we want to give some special attention here:
The most important constrain ─ with which readers may be familiar with to some degree ─ is the constrain of events being evident. The term evidence, as present in ‘evidence-based medicine’, is a science theoretic term and originated in its modern form from concepts such as phenomenology (e.g., Husserl). Roughly, data is considered to be evident, when it can potentially be experienced by anyhumaninference, regardless or independent of their (prior) beliefs. Formally this can be expressed as ‘intersubjectivity of human experience on an independent event’, such as a ball falling to the ground, which is an event that as such is not influenced by my thoughts, or what I wish to happen (are independent). Such an event can therefore potentially be perceived by others, again: independent of my thoughts, beliefs, intentions (therefore ‘intersubjective’, and in order to address the ‘tools’ used to infer: ‘human inference’).
Another way evidence as an attribute is commonly expressed is by saying “dataisgiven”. The phrase “data is given” is etymologically redundant, as data originated in the Latin language and also means “that what is given” ─ and stands in contrast to what is set in advance, i.e., our (hypo)theta, the prior hypothesis (the term hypothesis originated in Greek and means “to place under” ─ or setbefore in the sense of our temporal hierarchy of the three steps of scientific inquiry).
The above terminological use of the term ‘evident’ may appear confusing, considering the everyday use of the term evidence: Note that the practice of saying a study to have shown evidence in a belief or hypothesis often implicitly jumps from the prior constrain on data acquisition to the interpretation of the outcome of the hypothesis testing in the mathematical sense. Both involve evidence, either as a constrain, or as a possible interpretation of statistical results. Though this is still partially overlapping with the concept of the significanceof a model in an unfortunate way, as the significance is not the only marker for ‘evidence’ of any kind, as we will see. As the pandemic and developments in the recent years have often blurred the view on science massively, we belief that it is important to note here that evidence-based medicine is not just an advocacy on how to interpret the outcome of a (statistical) hypothesis test correctly (the significance of a model), but also a discourse on the evidence of inquiry as such. This approach to infer “on the world” therefore explicitly stands in contrast to mere belief systems of any kind, which may argue that a belief for itself leads to explicit knowledge of the world of some kind ─ e.g., the power, the effect, the impact, the existence of something ─ regardless any (“evident / given“) data that could prove the conditional relation between the hypothesis and the data. Such a violation of the steps of scientific inquiry can in some way intuitively be understood as tilting the temporal course of scientific inference in the sense that a prior hypothesis claims to already be the result of the evaluation of data inquiry, and may even claim to be the dataitself. There are other constraints, such as the validity of a test (does a test really test what it is supposed to test for) ─ however, for now these can be looked at as just further reflections on the same issue: gathering and inferring on evident data in a wider sense.
In comparison to our first step, consisting of our prior hypothesis only, the second step of “gathering data” can now be looked at as an actual or present relation between the data and our (hypo)theta. In probability theory this relation can be looked at logically as a conjunction, i.e., an “overlapping” between our hypothesis (theta) and the obtained data, where the overlapping indicates that both are true. True? This may be a little abstract, but the attribute true just says that when our prior hypothesis says “it is raining tomorrow” and the data shows that “it rained” the next day, then the event “it is raining” is true in both cases of theta and data. For theta anything we want can be pre-set as true ─ not so for data, as data is given in the sense that we have to gather it, make experiences, observe an event.
Those with prior knowledge may recall that the so-called null hypothesis refers to a case where the hypothesis is set to be false (theta or thetac, where the “c” stands for complement).
However, in probability theory the conjunction between data and theta is also called a joint (probability). Later we will add actual probabilities to the Venn diagram, suggesting that for any of the possible ‘joint combinations’, there is a certain probability ranging from 0 to 1 assigned to it. Technically this joint probability is what can be called a probabilisticmodel ─ a model of the relation of theta and data. However, our linear model in the next part of this tutorial series will mathematically not be exactly the same, as it is “not made out of” probability values, but out of the values of a measurement (e.g., tea drank within time). This is where hypothesis tests in the mathematical sense come in play, as a t-test can be seen as a method to obtain values (such as the t-value) that can be used to obtain a p-value for a linear model. In other words: there are ways to “look at” or evaluate the results of a linear model under the consideration of probabilistic relations.
We will get back to all of this in detail soon. For now, just hold on to the idea of an overlapping of our hypothesis and data that fits to it in the sense that the assumptions hold in both “areas” ─ theta and data both being true or ‘the case’.
1.3 Evaluating and adjusting the hypothesis to fit the data
The consequence of gathering data ─ or “making experiences” ─ is usually that we evaluate and eventually adjust our hypothesis to (better) fit the actual experience, the actual data. We have to say “usually” as it is unfortunately not a “self-evident” practice, given a high tendency to produce positive results in science (‘publication bias’, resulting in a lot of studies not even getting close to the third step of statistical inference, which is unfortunate in a lot of ways).
The result of our evaluation will eventually become our new prior hypothesis. The better our hypothesis, the better we are to predict future events. Note that adjusting or updating the hypothesis in the mathematical sense concerns the probability of a hypothesis, as we will see in the next chapter, not inventing a completely new thesis (e.g., no changes in the variables or so, which could be referred to as HARKing (Hypothesizing After the Results are Known), which is essentially faking a course of inference). However, in a wider sense, updating a hypothesis in terms of changing our beliefs still represents what we do as a consequence of gathering and evaluating data in the long run: we change/update our model of the world, gathering new insights over time (Bayes’ rule therefore represents “learning”, as we will see).
To give you another simple non-feathery example of what updating a hypothesis means, given a negative result: if we were to experience a ball to never fall down by itself, we would adjust or “doubt” on our hypothesis of gravitation in the long run.
Now that we have roughly gone through all the steps of hypothesis testing, let us look at them from a mathematical perspective.
We will leave a summary at the end of every chapter, to give you an overview of what we have learned so far:
2 Conditional probability and Bayes’ theorem
Translating our discussion above into mathematics is fairly easy. There are only two minor features we have to include into our intuition on hypothesis testing to make it work smoothly.
Categorical variables: The binary distinction true and false can be looked at as categorical distinction. Other categories are also possible, such as heads or tails. In the latter cases it depends on what you chose first as theta, either heads or tails, to translate the coin into a binary. However, there is no restriction to binary categories. The categories that were chosen are for themselves contingent, but of course still depend on pre-set decisions wemake (e.g., the number of categories). Below we will mostly work with binary outcome options.
Probability values: Our confidence in a hypothesis will be represented in the form of probability values. Before we were just working with the categorical distinction of true and false. Each of them will now just get assigned a probability between 0 and 1. This is importantly not the same as the binary distinction true and false, as these are just categories, such that there could be a 0.1 probability assigned to theta. However, for the addition of probabilities to our variables to be recognizable, our variables theta and data will now be symbolically marked by a P for ‘probability’ standing in front of them ─ such that our prior hypothesis, i.e., our hypothesis regardless any (new) data, will be written as P(theta).
Now that we are set, we can simply go through the same three steps again, including the mathematical symbols used to address what we have gone through conceptually already. It is also important to highlight the simple and intuitive temporal hierarchy, corresponding the steps.
I guess the mathematical denotation for the prior and the joint probability does not add much to what we have gone through so far, just the third step, the posteriorprobability should be symbolically new to us. Outspoken the posterior probability is read as “the probability of theta, given or under the condition of data”, where the sign ”|” means given or under the condition of. Therefore the name: conditional probability. The temporal course of our condition may appear inverted to you, and this is partially true! The posterior probability actually refers to the data being a ‘prior condition’ to the hypothesis this time, as the posterior probability reflects the hypothesis, after we have observed data. Before we argued that we startwith a hypothesis and thengatherdata, which was the transition from the first to the second step, not the second to the third step, as we will see.
Let us go one step back and fully forget about the posterior for now, in order to focus on what a joint probability actually is. So far, we referred to the second step as an overlapping or a joint between theta and data. Both, the joint probability and the conditional probability reflect a relation between two variables. However, the difference between a conditional probability, such as the posterior, and the joint probability though is simple: A joint probability is considered a probability where the specific course of conditions is not yet defined or decided, such that either “theta as prior condition for data” or “data as prior condition for theta” could be obtained from evaluating the joint (note that this refers to a prior only in terms of our temporal hierarchy, as the prior is usually denoted P(theta)).
The joint probability ─ at least the way it is denoted above ─ therefore reflects the probability of encountering theta and data in general, not under a particular condition of our temporal course of inference, so to speak. In another words, the course of inference is potentiallybidirectional: we could retrieve P(theta∣data) or the inverted P(data∣theta) when evaluating the same joint probability for itself. The conditional probability we end up with depends on the variable we chose to start with ─ either with P(theta) or P(data). Other than that, the steps are the same, also consistent with our ‘temporal hierarchy’, namely: starting with a “single probability” that is always “prior” in a wider sense, then moving to the joint, ending in a conditional probability. The order of variables is therefore not important when denoting joint probabilities, such that P(theta,data) and P(data,theta) are the same (again, the comma is in this case a mathematical symbol spoken “and” and refers to the overlapping, the conjunction between data and theta).
There is still another way of representing and especially calculating the joint probability ─ and this is where Bayes’ theorem comes in play. Note that Bayes’ rule and conditional probability are essentially the same. However, classic conditional probability does not reflect the joint probability by itself as the result of a weighted conditional probability that can also be represented via a decision tree, as we will see below (this is ─ as much as I know ─ the only real difference, apart from the historic remarks referring to Thomas Bayes, which we will not get into here). Bayes’ rule just extends conditional probability on that matter, so to speak.
To get a closer look at one specific way of obtaining the joint probability, let us zoom in into one particular chain of decisions made.
With the chain rule we have introduced two things: a mathematical algorithm to calculate the joint probability from one course of inference only, and the mentioned likelihood P(data∣theta) ─ another conditional probability.
To make sense of the likelihood: Remember Ms. Miranda, when she was on her way to make new observations? This is what the likelihood is actually about: gathering data, under the condition of theta, moving from step I to step II. The evaluation of the joint eventually resulted in the posterior, representing the inverse condition, moving from step II to step III ─ the probability of theta, after we observed data.
What Bayes’ rule does mathematically is what we literally just have gone through conceptually: the process of hypothesis testing more or less inverts the likelihood to become the posterior. “More or less” as this is numerically only the case under special prior conditions, as we will see, but it is still true: Bayes’ rule is method to invert the experience under the condition of a hypothesis to become the hypothesis under the condition of (new) experiences made (which eventually becomes the future prior and therefore influences or changes the next joint probability formed with the new prior, i.e., the model is updated).
To wrap this up: The chain rule above shows that the joint probability not only consists of the likelihood, moving from step I to step II, but also of the prior itself, which can importantly be understood as a weight in the form of an expectation on the likelihood. The likelihood again represents the observation, the experiences we make, our data, given our hypothesis (or the prob. of data after we formed a hypothesis). But what does a weighting actually mean? The concept is actually simple: something can be weighted to have a higher probability of occurrence in general (over time) as something else, nevertheless the observation (regardless any data!). In other words: a model can entail a prior “confidence” in a hypothesis which may be higher than the prior confidence into its complement and eventually influences how an event is evaluated a posteriori (after making an experience). If this sounds abstract, think of ‘classic conditioning’ in psychology as a classic example, where a conditioned weight, i.e., a prior expectation, influences the behavior/inference of a subject (build up on weighted inference) in relation to certain events (the behavior is therefore different to unconditioned subjects over time; in other words: classic Bayes’ rule represents a “learning algorithm”).
Note up front that a probability of .5 for theta and .5 for its complement refers to a special kind of “balanced prior” weight, which is also referred to as “uniform” prior ─ we will get there soon.
In order to fully make sense of all of the above, we can now finally discuss Bayes’ rule via actual mathematical formulas. Let us start with the fact that there are two approaches to obtain one and the same joint probability, since:
On the far-left we see the posterior multiplied or weighted by P(data). On the far right we see the likelihood weighted by the prior. Both multiplications lead to the same joint P(theta,data).
Now we know that Bayes’ rule can be understood as a way to obtain a joint probability from (weighted) conditional probabilities. Let us now first take the simplest route to Bayes’ theorem just by taking basic rules of equations into consideration. In order to obtain the posterior probability from the formula above, we could just divide the whole line of equation by P(data) to get rid of the term on the far-left side of the equation:
Just in case it appears confusing, the joint probability is crossed out too, as the result of our actions leaves us with a conditional probability only (deciding for the course of inference). We have now decided the course of inference so to speak. The last line eventually represents what is called Bayes’ rule.
An easy way to remember Bayes’ rule is to follow the variables clockwise, starting with the posterior, and chant them, going: theta-data // data-theta // theta --- data (with a little break in-between the prior and the model evidence, which stand for themselves). As it just repeats an inversion of the pair theta and data, it is not too hard to remember (also conceptually).
As mentioned, the posterior can be understood as the result of the jointbeing divided by the weight of the complementary or ‘conditionally inverted’ joint ─ a kind of “counter weight”, so to speak. To get a better intuition what this means, let us go back to our Venn diagram.
Our prior P(theta) will represent a weight to the likelihood, resulting in a joint probability. The variable P(data) on the other hand, also called the model evidence, works as a kind of counter weight to the joint as a whole (not just the likelihood), resulting in the posterior probability.
Another way of reflecting on the joint probabilities is via a conditional probability table, which also reveals a way how we can obtain P(B) or P(A), given the joint probability.
Note thatP(B)is a value that can also be provided, in order to correct or compare it with the assumed model evidence of just one event (or the events a model works with). You may have already found respective examples during your own research, where the model evidence is not calculated via summing out, but provided. A common example is weather forecast: What is the probability of tomorrow being sunny, say given that it is rainy today. In mathematical terms this would be denoted as: P(t+1=sunny∣t=rainy). In such a case of meteorology the general probability of a day being rainy, i.e., P(rainy), may have been externally obtained from an average on the data from a long-range observation and is not just calculated from the information given by the prior and the likelihood of just one or a few events (note that statistical thermodynamics / mechanics relies on the same idea of conditional probability, so it totally makes sense to predict weather with such a probabilistic model; we will provide tutorials on thermodynamics and information theory in the near future as well!).
Apart from that, Bayes’ rule is in general used to test a test, i.e., the evidence of a test ─ how well it predicts. It does so by considering a longer range of observations over time, or a wider population in the form of a general frequency of P(Event), and again not just the one or few events a model works with. The posterior can in such cases act as the positive predictive value (PPV), updating our confidence in a test. In essence such calculations rely on two conditional dimensions: the actualdata (set as hypothesis), i.e., true and false, and the predicteddata: positive and negative. The model evidence is here more clearly used to reflect on the evidence of a statistical model, i.e., our joint probability (there is a little more to say to the formula below, but we will do so some other time in another tutorial). The far-right refers to probabilities of the joint and the model evidence, to clearly relate it to Bayes rule’, even though the middle part of the equation does not yet indicate probabilities (is not normalized in this case).
That was it, you have now mastered the essentials of hypothesis testing reflected as conditional probability and Bayes’ rule respectively, both on the conceptual and on the mathematical level. In the next chapter we will do some calculations to gain some experience with applications of the math that we have learned so far using R. After that we will introduce the difference between the Bayesian and the frequentist interpretation of conditional probability aka Bayes’ rule and finally reveal what a p-value actually is (we have indirectly encountered before).
Below you see the posterior on the far-left side of the equation, conditional probability in the middle and Bayes’ rule on the far-right side of the equation.
3 Computing conditional probability / Bayes’ rule in R
After elaborations on the concept and the mathematics behind hypothesis testing (its mathematical logic), we will now do some calculations in R for a change (the application of mathematical logic). This will help us to get a clear orientation on what the results of the upper formula may look like and we will also numerically prove some assumption we made above. Note that using R will be much easier than it might sound, as we will be using it more as a simple calculator for now (note, all the following can also be done ‘by hand’ ─ also holds for the linear regression, as we will see). We will also introduce R in the second part of this tutorial series again, which also involves things as plotting ─ so no worries if this either appears too much, or too basic to you for now.
Below you will find code for the programming language R that can be used to calculate the posterior probability. Note again, if you are new to R you can either just read this tutorial and with it read the code ─ as we will provide the output for every line of commands ─ or you just download R and RStudio, open a new script and copy and paste the code provided below into it, or download and open the R script we provided below (or type it in yourself, if you wish to do so).
R script, corresponding to the tutorial:
The first lines of our script will be just a test and looks like this:
# This is a test, which will also be the name of the ‘object’
test = 2 + 5 # Execute this line!
Note that lines that start with #, as well as any code within a line after a # was placed, is considered a “comment” and will not be ‘understood as code’ by R (so you can also mark and execute it and it will not mess up anything). Otherwise text will be interpreted as code, such as a calculation ─ which will lead to (mostly enigmatic) errors presented in the console below the script (see figure below for details).
Mark the lines you want toexecute and press ALT+ENTER. You can also execute comments, so if a script is set as a whole, one can also mark all and then execute the script as whole. The result can be seen in the environmenttab on the upper right side of RStudio (see figure below). If you ever feel that your script is presenting a funny output (especially after a series of errors), clear the environment via thebrush tool ─ there is another brush-icon placed in the console tab to clear the console. Now mark the name of an object onlyand press ALT+ENTER again to obtain the results in the console (below the script) ─ you won’t need to know much more for this tutorial for now, believe us!
The consequence of your actions should result in the following console output (ignore the  for a moment).
# Console output:
#  7
Note that we can represent our probability variables either as single value or as a probability vector that sums to 1, such that we will always consider theta and its complement at the same time. We will start with using single values first.
The following example will refer to anything you consider a theta and for which (evident) data is given. For a simple example, we will start with flipping a coin, arguing that the coin is fair, such that a 50%/50% chance is given to encounter either heads or tails (or 0 or 1; true or false…). As this is an example, we can provide ourselves with data and set the likelihood for ourself. Recall that we need the likelihood to calculate the joint probability via Bayes’ rule and that the likelihood is a conditional probability for itself ─ reflecting on the transition of step I to step II within hypothesis testing (I. forming a hypothesis. II. making (new) experiences / gathering data). The likelihood can therefore be reflected as the probability of data, after we have formed a hypothesis (in contrast to the inverted posterior, reflecting moving from step II to step III: ─ expressing the probability of the theta, after data was obtained).
# Define you prior, e.g., .5 for heads.
# Note that R is a case sensitive language (“prior” not same as “Prior”).
prior = .5
likelihood = .5
Now we can define the likelihood corresponding to our prior hypothesis. It can either be .5 again, which would represent a truly fair coin (after some rounds of flipping the coin) ─ and suggests that our hypothesis holds. Or you assign a value of .6 or any other value deviating from .5, which would suggest that the coin is phony and that our prior assumptions about the coin are wrong.
Either way, we can now calculate the joint probability, as well as the model evidence:
In this case the prior hypothesis was notreally updated, such that prior and posterior remain the same! What if the likelihood or prior changes?
To get a better overview of the process above, we are now going to slightly expand the math: Next we are not only using probability values, but probability vectors to do Bayes’, which lets us calculate both theta and theta at the same time. To represent our vectors in R, we will use the combine function c(), with which objects of any kind can be combined as a list of values, objects or even a list of lists (note that there is also a function called list() as well, which is not structured in rows and columns, but sequentially numerates elements ─ also of various kinds of classes (vector, matrix, single values, whole data frames, all in one list)).
However, the upper formula and code using probability vectors just slightly differs from what we have gone through so far. Note that for non-binary outcomes the vector would just be expanded holding 3+ values. The prior probability distribution you see below is also referred to as uniform prior (will be important in a bit). Let us first go through the formulas and then do some computation with R. Play around with the input values and you might notice a special characteristic of the posterior for yourself, when changing the values of the likelihood, but keeping the prior equally distributed (uniform). The formula below formally does not include the complement, but it does so by using the probability vectors. Keep in mind that every vector has to sum to 1, when changing the input values below. Also keep in mind that if you change a line you have to execute the respective line and every line involved again to obtain the new results.
Above we have calculate the model_evidence using the sum() function. Basically, this function does what is says and uses an input, such as our joint probability vector with two values ─ .25 and .25 ─ and sums up every element of that vector (we will further get into what functions are in general in the next part of this tutorial series). Keep in mind that an R script is executed from top to bottom. The R script we provided can theoretically be executed as a whole (mark all and execute). However, it may be that a variable, e.g., with the name prior gets redefined in lower parts of a script (changes in the environment!). In other words: The content of the previous object with the same name prior will be “overwritten” so to speak.
Have you figured out what happened, when using a uniform prior, changing the likelihood only? You are right! The posterior will always match the likelihood. How is this possible? The reason is that in such a case the weight and the “counter weight” eliminate each other. Let us take a look at the formula to understand what that means mathematically:
The above can also be simplified to the following, where P(A) and P(B) eliminate each other under certain conditions:
With this we have already revealed the most essential difference between the frequentist and Bayesian interpretation of Bayes’ rule, as the frequentist always assumes a uniform prior, such that the posterior will be equivalent to the maximum likelihood estimate (to which we will come in the third part of this tutorial series in detail).
However, the likelihood P(data∣theta) represents what is called the alternative hypothesis and is therefore almost what is typically referred to as the p-values, testing for the null hypothesis ─ which would be denoted as the probability of data given theta, i.e., P(data∣theta). In other words: the probability of having data saying something to be true, where the hypothesis actually rejects it. Doing your own research, you may come across phrases such as that “the p-value is the probability of the data given that the null hypothesis is true” ─ don´t get confused, it is the same as saying theta to be false. There are also some other minor twists concerning the t-test, the confidence interval, the power of a study etc. that we have to go through when trying to fully understand how we get to a p-value given a linear regression model (again this will be done in the third part of this series). However, we are still well prepared to discuss the difference between the Bayesian and the frequentist approach to probability in the next chapter.
4 The Bayesian and the frequentist approach to (conditional) probability
How does the special case of equivalence between prior and likelihood fit our intuition? In general, a uniform prior can be considered as taking a “neutral position a priori” ─ at least neutral regarding the weight of the pre-set contingencies of our categories (above it was binary). Equivalent to our coin example we assume a kind of ‘fairness’ a priori. This makes sense in a lot of ways ─ in others it absolutely doesn´t and contradicts a neutral position due to ‘false balance’, due to a uniform prior. In order to get a grip on what that means, we will have a look at another famous xkcd webcomic:
There is more to overfitting and we will probably come up with a tutorial on this in the future too. The take home message that we intended to convey is that both interpretations of Bayes’ rule / conditional probability lead to problems in similar forms, when trying to gain evidence of any kind from a statistical analysis. The Bayesian approach is trying to get hold of issues such as overfitting, by setting informed priors (e.g., on prior knowledge from previous research). The frequentist approach will have similar issues, just more related to the interpretation of the results and less to the way data is gathered (when likelihood is weighted). There is a great number of methods trying to overlook and overcome such boundaries in any of the two “fields”, both mathematically and intellectually (‘What is evidence?’). At the end, the attribute “fields” is somewhat over the top, as both approaches refer to the same equation, just under different prior assumptions.
In general, the difference appears rather synthetic, as both refer to the same formula, the same steps of hypothesis testing, just under different prior distributions. The tilting effect between likelihood and posterior in combination with the ex negativo likelihood P(B∣A), when testing for the null hypothesis, eventually blurs away the fact how banal the difference between the two approaches is in the end.
When doing your own research, you may have come across further distinctions between the Bayesian and the frequentist approach. E.g., in Bayesian statistics it is said that the hypothesis is dynamic or changes, the data being something constant. In frequentist statistics it is said that the data or likelihood changes, and the hypotheses stay stable (i.e., binary (0% and 100%). This may seem complicated or even enigmatic, but it essentially just refers to the denotation of the variables:
As mentioned, the difference between the Bayesian and frequentist approach is often cast as philosophical discussion on probability as such: Bayes’ rule assumes that experience changes the way we hypothesize or expect the world to be a priori in the future (posterior becoming the new prior), where the frequentist approach assumes the possibility of a stable neutral position a priori and casts evidence as a frequency of an assumption (being able to “frequently discard” the null hypothesis). Though, arguing this to be a “philosophical” discussion is somewhat misleading, as we learned that the above is again just a reflection on Bayes’ rule (in particular likelihood and posterior). In general, just keep in mind that the frequentist approach to probability just reflects a special case of Bayes’ theorem.
However, being aware of different approaches to probability theory does not involve choosing for a specific side in that discourse. We rather believe it to be essential to be aware of the prior considerations of hypothesis testing no matter how the chosen prior weight may look like. Still, in a lot of cases it is not just a matter of style or opinion, which side or method we choose, as we are all Bayes’ when it comes to, e.g., the positive predictive values, or when performing differential diagnostics (“investigative reasoning”, see below).
One last thing before we close our reflections on Bayes and frequentist stats: As mentioned, the Bayesian analogue of a “frequency” is the posterior to become the prior. This can be understood as a structural recursion / iteration of hypothesis testing. Investigative reasoning has therefore often been related to Bayes’ rule: Vanessa Holmes’ new case involves four suspects, one of them being the murder of an innocent racoon. A prior probability could for now look something like [.18.104.22.168] for each suspect, when there are no specific prior assumptions on a suspect given so far (no clues) ─ staying elementary neutral and letting the facts speak first, so to speak (initial uniform prior). Holmes checked on one of the suspects, but the possible suspect has a clear alibi. With this new observation, our prior probabilities will subsequently change to [.33 .33 .33 .0], so our model (joint) was subsequently updated (as it consists of prior and likelihood). Another example would be reasoning in the medical field in terms of differential diagnostics, as well as evidence-based medicine in general, as we now know. Here is some code to replicate the Vanessa Holmes example:
The first part of our series on statistical inference has come to an end. We hope that this has given you a stable overview over what hypothesis testing is in general all about and how it is related to conditional probability. You will definitely come across conditional probability in various forms, when further digging into statistics. As a lot of our section members are also interested in computational neuroscience, bioinformatics and data science in general. We will therefore soon also provide tutorials on topics such as information theory (Shannon, Akaike; both relies on reflections thermodynamic analogies, which also involves conditional probability (Boltzmann and Gibbs´ entropy, free energy etc.)), predictive processing / active inference (disconnection hypothesis arguing schizophrenia to be a weighting problem) and many more, which all rely on the basic concept of hypothesis testing in the sense of Bayes’ rule (tutorials on these topics are also just about to be finished, so stay tuned!).
The information theoretic use of conditional probability is one of the most fascinating and mind blowing and refers to the “bit” as a statistical quantity, showing that communication is a stochastic process and comes without conveying meaning in the actual sense (Shannon). Something that especially research in the field of humanities struggled a lot with conceptually when trying to understand information technology (unfortunately there a lot of heavily faulty interpretations of information theory and what a computer actually does).
However, note that hypothesis testing is often also referred to as abductive inferencein contrast to deductive and inductive reasoning and was defined by the mathematician and philosopher C.S. Peirce that had a great influence on the development of information theory (logic gates, further development of Boolean algebra, abductive inference and communication; the temporal triad we referred to is essentially referring to the triadic structure of abductive inference (related to semiotics)). The difference to deductive and inductive reasoning is essentially that abductive inference / hypothesis testing involves developing and evaluating something new, a hypothesis, and does not just evaluate a pre-set part-to-whole relation between a rule and a single event as in the other two forms of inference. Arguing that this can be looked at as a hierarchical and recursive process, such that a deduction is just an abduction of an abduction, then one can say that statistical reasoning in the sense of a mathematical method can be referred to as deductive reasoning (also under “neutral assumptions” in particular), as a process of forming a hypothesis has already been performed cognitively before and therefore on a lower-level of the hierarchy so to speak. However, it does not change our general intuition on hypothesis testing, it rather expands its influence and the possible complexity of representing argumentations in the form of statistical inference.
To also share some other submissions for the Summer of Math Exposition 2, I want to draw attention to the article by Jeffrey Wang that beautifully shows what it can cost others when medical doctors and others around them do not properly understand basic conditional probability.
SoME2 Reviewer and statistician here. Good job, it looks like you put a lot of effort into it. I however find it is still heavy in the sense that there are a lot of allusions and parenthesis, and most of the discussion is philosophical rather than mathematical (but you need the bases, right?) I saw that you talked of hypothesis testing while introducing mostly a bayesian approach: this can be confusing for a reader that will then search the literature and find that it is mostly a frequentist term.
Thanks so much for your feedback!! As you are our first external commentator within this collection, I want to thank you for contributing to our journal by making use of our open feedback options!! I will address your points in the next release, especially possible expectations from a frequentist approach. It is true that the tutorial rather focuses on cond. prob. / Bayes’ rule in general. It was also the intention of this tutorial to make readers aware of prior assumptions of the frequentist approach. The idea was also to show that an intuitive triadic structure of the process (set prior, gather data, update model / evaluate) holds in both approaches to probability, as the posterior under uniform considerations is just equivalent to the likelihood, not substantially different. However, it is true that it will probably help to make at least some more explicit introductory comments on the matter of expectation. The second and third part will especially focuses on a frequentist approach (lin. Reg. via linear least square method; t-test, F-statistic… the output of the summary(lm()) function in R). Thanks again! This really helps us a lot!