# Chapter 3 Probability

Probability theory is the branch of mathematics that deals with chance and uncertainty. It forms an important part of the foundation for statistics, because it provides us with the mathematical tools to describe uncertain events. The study of probability arose in part due to interest in understanding games of chance, like cards or dice. These games provide useful examples of many statistical concepts, because when we repeat these games the likelihood of different outcomes remains (mostly) the same. However, there are deep questions about the meaning of probability that we will not address here; see Suggested Readings at the end if you are interested in learning more about this fascinating topic and its history.

## 3.1 What is probability?

Informally, we usually think of probability as a number that describes the likelihood of some event occurring, which ranges from zero (impossibility) to one (certainty). Sometimes probabilities will instead be expressed in percentages, which range from zero to one hundred, as when the weather forecast predicts a twenty percent chance of rain today. In each case, these numbers are expressing how likely that particular event is.

To formalize probability theory, we first need to define a few terms:

• An experiment is any activity that produces or observes an outcome. Examples are flipping a coin, rolling a 6-sided die, or trying a new route to work to see if it’s faster than the old route.
• The sample space is the set of possible outcomes for an experiment. For a coin flip, the sample space is {H,T} where the brackets represent the sample space and H/T represent heads/tails respectively. For the die, the sample space is {1,2,3,4,5,6}. For the amount of time it takes to get to work, the sample space is all possible real numbers greater than zero (since it can’t take a negative amount of time to get somewhere, at least not yet).
• An event is a subset of the sample space. Here we will focus primarily on elementary events which consist of exactly one possible outcome, such as heads in a coin flip, a roll of 4 in dice, or 21 minutes to get home by the new route.

Now that we have those definitions, we can outline the formal features of a probability, which were first defined by the Russian mathematician Andrei Kolmogorov. If $$P(X_i)$$ is the probability of event $$X_i$$:

• Probability cannot be negative: $$P(X_i) \ge 0$$
• The total probability of all outcomes in the sample space is 1. We can express this using the summation symbol $$\sum$$: $\sum_{i=1}^N{P(X_i)} = P(X_1) + P(X_2) + ... + P(X_N) = 1$

This is interpreted as saying “Take all of the N elementary events, which we have labeled from 1 to N, and add up their probabilities. These must sum to one.” An implication of this is that the probability of any individual event cannot be greater than one: $$P(X_i)\le 1$$

## 3.2 How do we determine probabilities?

Now that we know what a probability is, how do we actually figure out what the probability is for any particular event?

### 3.2.1 Personal opinion

Let’s say that I asked you what the probability was that Bernie Sanders would have won the US Presidential Election in 2016 if he had gained the Democratic nomination instead of Hillary Clinton. Here the sample space is {Sanders wins, Sanders loses}, but we can’t actually do the experiment to find the outcome. However, most people with knowledge of the election would be willing to offer a guess at the probability of this event. In many cases personal knowledge and/or opinion is the only guide we have determining the probability of an event, but this is not very scientifically satisfying.

### 3.2.2 Empirical frequency

Another way to determine the probability of an event is to do the experiment many times and count how often each event happens. From the relative frequency of the different outcomes, we can compute the probability of each. For example, let’s say that we are interested in knowing the probability of rain in San Francisco. We first have to define the experiment — let’s say that we will look at the National Weather Service data for each day in 2017 (which can be downloaded from https://www.ncdc.noaa.gov/) and determine whether there was any rain at the downtown San Francisco weather station.

# load data on rain in San Francisco and compute probability

# create a new variable indicating whether it rained on each day
SFrain <-
SFrain %>%
mutate(rainToday = as.integer(PRCP > 0))

SFrain_summary <-
SFrain %>%
summarize(
nRainyDays = sum(rainToday),
nDaysMeasured = n(),
pRainInSF = nRainyDays / nDaysMeasured
)

pander(SFrain_summary)
nRainyDays nDaysMeasured pRainInSF
73 365 0.2

According to these data, in 2017 there were 73 rainy days. To compute the probability of rain in San Francisco, we simply divide the number of rainy days by the number of days counted (365), giving P(rain in SF in 2017)=0.2.

How do we know that empirical probability gives us the right number? The answer to this question comes from the law of large numbers, which shows that the empirical probability will approach the true probability as the sample size increases. We can see this by simulating a large number of coin flips, and looking at our estimate of the probability of heads after each flip. We will spend much more time discussing simulation in a later chapter; for now, just assume that we have a computational way to generate a random outcome for each coin flip.

Figure 3.1 shows that as the number of samples (i.e., coin flip trials) increases, the estimated probability of heads converges onto the true value of 0.5. However, note that the estimates can be very far off from the true value when the sample sizes are small. A real-world example of this was seen in the 2017 special election for the US Senate in Georgia, which pitted the Republican Roy Moore against Democrat Doug Jones. Figure 3.2 shows the relative amount of the vote reported for each of the candidates over the course of the evening, as an increasing number of ballots were counted. Early in the evening the vote counts were especially volatile, swinging from a large initial lead for Jones to a long period where Moore had the lead, until finally Jones took the lead to win the race.

These two examples show that while large samples will ultimately converge on the true probability, the results with small samples can be far off. Unfortunately, many people forget this and overinterpret results from small samples. This was referred to as the law of small numbers by the psychologists Danny Kahneman and Amos Tversky, who showed that people (even trained researchers) often behave as if the law of large numbers applies even to small samples, giving too much credence to results from small datasets. We will see examples throughout the course of just how unstable statistical results can be when they are generated on the basis of small samples.

### 3.2.3 Classical probability

It’s unlikely that any of us has ever flipped a coin tens of thousands of times, but we are nonetheless willing to believe that the probability of flipping heads is 0.5. This reflects the use of yet another approach to computing probabilities, which we refer to as classical probability. In this approach, we compute the probability directly based on our knowledge of the situation.

Classical probability arose from the study of games of chance such as dice and cards. A famous example arose from a problem encountered by a French gambler who went by the name of Chevalier de Méré. de Méré played two different dice games: In the first he bet on the chance of at least one six on four rolls of a six-sided die, while in the second he bet on the chance of at least one double-six on 24 rolls of two dice. He expected to win money on both of these gambles, but he found that while on average he won money on the first gamble, he actually lost money on average when he played the second gamble many times. To understand this he turned to his friend, the mathematician Blaise Pascal, who is now recognized as one of the founders of probability theory.

How can we understand this question using probability theory? In classical probability, we start with the assumption that all of the elementary events in the sample space are equally likely; that is, when you roll a die, all of the possible outcomes ({1,2,3,4,5,6}) are equally likely to occur. (No loaded dice allowed!) Given this, we can compute the probability of any individual outcome as:

$P(outcome_i) = \frac{1}{\text{number of possible outcomes}}$

For the six-sided die, the probability of each individual outcome is 1/6.

This is nice, but de Méré was interested in more complex events, like what happens on multiple dice throws. How do we compute the probability of a complex event (which is a union of single events), like rolling a one on the first or the second throw? de Méré thought (incorrectly, as we will see below) that he could simply add together the probabilities of the individual events to compute the probability of the combined event, meaning that the probability of rolling a one on the first or second roll would be computed as follows:

$P(Roll1_{throw1} \cup Roll1_{throw2}) = P(Roll1_{throw1}) + P(Roll1_{throw2}) = 1/6 + 1/6 = 1/3$

De Méré reasoned based on this that the probability of at least one six in four rolls was the sum of the probabilities on each of the individual throws: $$4*\frac{1}{6}=\frac{2}{3}$$. Similarly, he reasoned that since the probability of a double-six in throws of dice is 1/36, then the probability of at least one double-six on 24 rolls of two dice would be $$24*\frac{1}{36}=\frac{2}{3}$$. Yet, while he consistently won money on the first bet, he lost money on the second bet. What gives?

To understand de Méré’s error, we need to introduce some of the rules of probability theory. The first is the rule of subtraction, which says that:

$P(\bar{A}) = 1 - P(A)$

where $$\bar{A}$$ means “not A”. This rule derives directly from the axioms that we discussed above; since A and $$\bar{A}$$ are the only possible outcomes, then their total probability must sum to 1. For example, if the probability of rolling a one in a single throw is $$\frac{1}{6}$$, then the probability of rolling anything other than a one is $$\frac{5}{6}$$.

A second rule tells us how to compute the probability of a conjoint event – that is, the probability of both of two events occurring. This version of the rule tells us how to compute this quantity in the special case when the two events are independent from one another; we will learn later exactly what the concept of independence means, but for now we can just take it for granted that the two die throws are independent events.

$P(A \cap B) = P(A) * P(B)\ \text{if and only if A and B are independent}$ Thus, the probability of throwing a six on each of two rolls is $$\frac{1}{6}*\frac{1}{6}=\frac{1}{36}$$.

The third rule tells us how to add together probabilities - and it is here that we see the source of de Méré’s error. The addition rule tells us that:

$P(A \cup B) = P(A) + P(B) - P(A \cap B)$ That is, the probability of A or B occurring is determined by adding together the individual probabilities, but then subtracting the likelihood of both occurring together. In a sense, this prevents us from counting those instances twice. Let’s say that we want to find the probability of rolling 6 on either of two throws. According to our rules:

$P(Roll1_{throw1} \cup Roll1_{throw2}) = P(Roll1_{throw1}) + P(Roll1_{throw2}) - P(Roll1_{throw1} \cap Roll1_{throw2})$ $= \frac{1}{6} + \frac{1}{6} - \frac{1}{36} = \frac{11}{36}$

Let’s use a graphical depiction to get a different view of this rule. Figure 3.3 shows a matrix representing all possible throws, and highlights the cells that involve a one on either the first or second throw. If you count up the cells in light blue you will see that there are 11 such cells. This shows why the addition rule gives a different answer from de Méré’s; if we were to simply add together the probabilities for the two throws as he did, then we would count (1,1) towards both, when it should really only be counted once.

### 3.2.4 Solving de Méré’s problem

Blaise Pascal used the rules of probability to come up with a solution to de Méré’s problem. First, he realized that computing the probability of at least one event out of a combination was tricky, whereas computing the probability that something does not occur across several events is relatively easy – it’s just the product of the probabilities of the individual events. Thus, rather than computing the probability of at least one six in four rolls, he instead computed the probability of no sixes across all rolls:

$P(\text{no sixes in four rolls}) = \frac{5}{6}*\frac{5}{6}*\frac{5}{6}*\frac{5}{6}=\bigg(\frac{5}{6}\bigg)^4=0.482$

He then used the fact that the probability of no sixes in four rolls is the complement of at least one six in four rolls (thus they must sum to one), and used the rule of subtraction to compute the probability of interest:

$P(\text{at least one six in four rolls}) = 1 - \bigg(\frac{5}{6}\bigg)^4=0.517$

de Méré’s gamble that he would throw at least one six in four rolls has a probability of greater than 0.5, explaning why de Méré made money on this bet on average.

But what about de Méré’s second bet? Pascal used the same trick:

$P(\text{no double six in 24 rolls}) = \bigg(\frac{35}{36}\bigg)^{24}=0.509$ $P(\text{at least one double six in 24 rolls}) = 1 - \bigg(\frac{35}{36}\bigg)^{24}=0.491$

The probability of this outcome was slightly below 0.5, showing why de Méré lost money on average on this bet.

## 3.3 Probability distributions

We often want to be able to quantify the probability of any possible value in an experiment. For example, on Jan 20 2018, the basketball player Steph Curry hit only 2 out of 4 free throws in a game against the Houston Rockets. We know that Curry’s overall probability of hitting free throws across the entire season was 0.91, so it seems pretty unlikely that he would hit only 50% of his free throws in a game, but exactly how unlikely is it? We can determine this using a theoretical probability distribution; during this course we will encounter a number of these probability distributions, each of which is appropriate to describe different types of data. In this case, we use the binomial distribution, which provides a way to compute the probability of some number of successes out of a number of “Bernoulli trials” (i.e. trials on which there is either success or failure and nothing in between) given some known probability of success on each trial. This distribution is defined as:

$P(k; n,p) = P(X=k) = \binom{n}{k} p^k(1-p)^{n-k}$

This refers to the probability of k successes on n trials when the probability of success is p. You may not be familiar with $$\binom{n}{k}$$, which is referred to as the binomial coefficient. The binomial coefficient is also referred to as “n-choose-k” because it describes the number of different ways that one can choose k items out of n total items. The binomial coefficient is computed as:

$\binom{n}{k} = \frac{n!}{k!(n-k)!}$ where the explanation point (!) refers to the factorial of the number:

$n! = \prod_{i=1}^n i = n*(n-1)*...*2*1$

In the example of Steph Curry’s free throws:

$P(2;4,0.91) = \binom{4}{2} 0.91^2(1-0.91)^{4-2} = 0.040$

This shows that given Curry’s overall free throw percentage, it is very unlikely that he would hit only 2 out of 4 free throws. Which just goes to show that unlikely things do actually happen in the real world.

### 3.3.1 Cumulative probability distributions

Often we want to know not just how likely a specific value is, but how likely it is to find a value that is as extreme or more than a particular value. To answer this question, we can use a cumulative probability distribution; whereas a standard probability distribution tells us the probability of some specific value, the cumulative distribution tells us the probability of a value as large or larger (or as small or smaller) than some specific value.

In the free throw example, we might want to know: What is the probability that Steph Curry hits 2 or fewer free throws out of four, given his overall free throw probability of 0.91. To determine this, we could simply use the the binomial probability equation and plug in all of the possible values of k:

$P(k\le2)= P(k=2) + P(k=1) + P(k=0) = 6e^{-5} + .002 + .040 = .043$

In many cases the number of possible outcomes would be too large for us to compute the cumulative probability by enumerating all possible values; fortunately, it can be computed directly. For the binomial, we can do this in R using the pbinom() function:

# compute cumulative probability distribution for Curry's free throws

tibble(
numSuccesses = seq(0, 4)
) %>%
mutate(
probability = pbinom(numSuccesses, size = 4, prob = 0.91)
) %>%
pander()
numSuccesses probability
0 0
1 0.003
2 0.043
3 0.314
4 1

From this we can see that the probability of Curry landing 2 or fewer free throws out of 4 attempts is 0.043.

## 3.4 Conditional probability

So far we have limited ourselves to simple probabilities - that is, the probability of a single event or combination of events. However, we often wish to determine the probability of some event given that some other event has occurred, which are known as conditional probabilities.

Let’s take the 2016 US Presidential election as an example. There are two simple probabilities that we could use to describe the electorate. First, we know the probability that a voter in the US affiliated with the Republican party: $$p(Republican) = 0.44$$. We also know the probability that a voter cast their vote in favor of Donald Trump: $$p(Trump voter)=0.46$$. However, let’s say that we want to know the following: What is the probability that a person cast their vote for Donald Trump, given that they are a Republican?

To compute the conditional probability of A given B (which we write as $$P(A|B)$$, “probability of A, given B”), we need to know the joint probability (that is, the probability A and B) as well as the overall probability of B:

$P(A|B) = \frac{P(A \cap B)}{P(B)}$

That is, we want to know the probability that both things are true, given that the one being conditioned upon is true.

It can be useful to think of this is graphically. Figure 3.4 shows a flow chart depicting how the full population of voters breaks down into Republicans and Democrats, and how the conditional probability (conditioning on party) further breaks down the members of each party according to their vote.

## 3.5 Computing conditional probabilities from data

For many examples in this course we will use data obtained from the National Health and Nutrition Examination Survey (NHANES). NHANES is a large ongoing study organized by the US Centers for Disease Control that is designed to provide an overall picture of the health and nutritional status of both adults and children in the US. Every year, the survey examines a sample of about 5000 people across the US using both interviews and physical and medical tests. The NHANES data is included as a package in R, making it easy to access and work with. It also provides us with a large, realistic dataset that will serve as an example for many different statistical tools.

Let’s say that we are interested in the following question: What is the probability that someone has diabetes, given that they are not physically active? – that is, $$P(diabetes|inactive)$$. NHANES records two variables that address the two parts of this question. The first (Diabetes) asks whether the person has ever been told that they have diabetes, and the second (PhysActive) records whether the person engages in sports, fitness, or recreational activities that are at least of moderate intensity. Let’s first compute the simple probabilities.

# Summarize NHANES data for diabetes and physical activity

# drop duplicated IDs within the NHANES dataset
NHANES_diabetes_activity <-
NHANES %>%
distinct(ID, .keep_all = TRUE) %>%
drop_na(PhysActive, Diabetes)

pander('Summary data for diabetes')

Summary data for diabetes

NHANES_diabetes_activity %>%
count(Diabetes) %>%
mutate(
prob = n / sum(n)
) %>%
pander()
Diabetes n prob
No 4893 0.899
Yes 550 0.101
pander('Summary data for physical activity')

Summary data for physical activity

NHANES_diabetes_activity %>%
count(PhysActive) %>%
mutate(
prob = n / sum(n)
) %>%
pander()
PhysActive n prob
No 2472 0.454
Yes 2971 0.546

This shows that the probability that someone in the NHANES dataset has diabetes is .101, and the probability that someone is inactive is .454.

To compute $$P(diabetes|inactive)$$ we would also need to know the joint probability of being diabetic and inactive, in addition to the simple probabilities of each:

# compute joint probabilities for diabetes and physical activity

NHANES_diabetes_stats_by_activity <-
NHANES_diabetes_activity %>%
count(Diabetes, PhysActive) %>%
mutate(
prob = n / sum(n)
)

pander(NHANES_diabetes_stats_by_activity)
Diabetes PhysActive n prob
No No 2123 0.39
No Yes 2770 0.509
Yes No 349 0.064
Yes Yes 201 0.037

Based on these joint probabilities, we can compute $$P(diabetes|inactive)$$:

# compute conditional probability p(diabetes|inactive)

P_inactive <-
NHANES_diabetes_activity %>%
summarise(
mean(PhysActive == "No")
) %>%
pull()

P_diabetes_and_inactive <-
NHANES_diabetes_stats_by_activity %>%
dplyr::filter(Diabetes == "Yes", PhysActive == "No") %>%
pull(prob)

P_diabetes_given_inactive <-
P_diabetes_and_inactive / P_inactive

P_diabetes_given_inactive
## [1] 0.1411812

The first line of code in this chunk computed $$P(inactive)$$ by taking the mean of a test for whether the PhysActive variable was equal to “No” for each indivdual. This trick works because TRUE/FALSE values are treated as 1/0 respectively by R; thus, if we want to know the probaility of some event, we can generate a Boolean variable that tests for that event, and then simply take the mean of that variable. We then use that value to compute the conditional probability, where we find that the probability of someone having diabetes given that they are physically inactive is 0.141.

## 3.6 Independence

The term “independent” has a very specific meaning in statistics, which is somewhat different from the common usage of the term. Statistical independence between two variables means that knowing the value of one variable doesn’t tell us anything about the value of the other. This can be expressed as:

$P(A|B) = P(A)$

That is, the probability of A given some value of B is just the same as the overall probability of A. Looking at it this way, we see that many cases of what we would call “independence” in the world are not actually statistically independent. For example, there is currently a move by a small group of California citizens to declare a new independent state called Jefferson, which would comprise a number of counties in northern California and Oregon. If this were to happen, then the probability that a current California resident would now live in the state of Jefferson would be $$P(\text{Jefferson})=0.014$$, whereas the proability that they would remain a California resident would be $$P(\text{California})=0.986$$. The new states might be politically independent, but they would not be statistically independent, because $$P(\text{California|Jefferson}) = 0$$! That is, while independence in common language often refers to sets that are exclusive, statistical independence refers to the case where one cannot predict anything about one variable from the value of another variable. For example, knowing a person’s hair color is unlikely to tell you whether they prefer chocolate or strawberry ice cream.

Let’s look at another example, using the NHANES data: Are physical health and mental health independent of one another? NHANES includes two relevant questions: PhysActive, which asks whether the individual is physically active, and DaysMentHlthBad, which asks how many days out of the last 30 that the individual experienced bad mental health. We will define a new variable called badMentalHealth as having more than 7 days of bad mental health in the last month, and then determine whether they are independent by asking whether the simple probability of bad mental health is different from the conditional probability of bad mental health given that one is physically active.

# compute probabilities for mental health and physical activity
NHANES %>%
dplyr::filter(
Age >= 18,
!is.na(PhysActive),
) %>%

NHANES_MentalHealth_summary <-

pander(NHANES_MentalHealth_summary)
0.164
NHANES_MentalHealth_by_PhysActive <-
group_by(PhysActive) %>%

pander(NHANES_MentalHealth_by_PhysActive)
No 0.2
Yes 0.132

From this we see that $$P(\text{bad mental health})$$ is 0.1640567 while $$P(\text{bad mental health|physically active})$$ is 0.1320808. Thus, it seems that the conditional probability is somewhat smaller than the overall probability, suggesting that they are not independent, though we can’t know for sure just by looking at the numbers. Later in the course we will encounter tools that will let us more directly quantify whether two variables are independent.

## 3.7 Reversing a conditional probability: Bayes’ rule

In many cases, we know $$P(A|B)$$ but we really want to know $$P(B|A)$$. This commonly occurs in medical screening, where we know $$P(\text{positive test result|disease})$$ but what we want to know is $$P(\text{disease|positive test result})$$. For example, some doctors recommend that men over the age of 50 undergo screening using a test called prostate specific antigen (PSA) to screen for possible prostate cancer. Before a test is approved for use in medical practice, the manufacturer needs to test two aspects of the test’s performance. First, they need to show how sensitive it is – that is, how likely is it to find the disease when it is present: $$\text{sensitivity} = P(\text{positive test| disease})$$. They also need to show how specific it is: that is, how likely is it to give a negative result when there is no disease present: $$\text{specificity} = P(\text{negative test|no disease})$$. For the PSA test, we know that sensitivity is about 80% and specificity is about 70%. However, these don’t answer the question that the physician wants to answer for any particular patient: what is the likelihood that they actually have cancer, given that the test comes back positive? This requires that we reverse the conditional probability that defines sensitivity: instead of $$P(positive\ test| disease)$$ we want to know $$P(disease|positive\ test)$$.

In order to reverse a conditional probability, we can use Bayes’ rule:

$P(B|A) = \frac{P(A|B)*P(B)}{P(A)}$

Bayes’ rule is fairly easy to derive, based on the rules of probability that we learned earlier in the chapter. First, remember the rule for computing a conditional probability:

$P(A|B) = \frac{P(A \cap B)}{P(B)}$

We can rearrange this to get the formula to compute the joint probability using the conditional:

$P(A \cap B) = P(A|B) * P(B)$

Using this we can compute the inverse probability:

$P(B|A) = \frac{P(A \cap B)}{P(A)} = \frac{P(A|B)*P(B)}{P(A)}$

If we have only two outcomes, we can express this in a somewhat clearer way, using the sum rule to redefine $$P(A)$$:

$P(A) = P(A|B)*P(B) + P(A|\neg B)*P(\neg B)$

Using this, we can redefine Bayes’s rule:

$P(B|A) = \frac{P(A|B)*P(B)}{P(A|B)*P(B) + P(A|\neg B)*P(\neg B)}$

We can plug the relevant numbers into this equation to determine the likelihood that an individual with a positive PSA result actually has cancer – but note that in order to do this, we also need to know the overall probability of cancer in the person, which we often refer to as the base rate. Let’s take a 60 year old man, for whom the probability of prostate cancer in the next 10 years is $$P(cancer)=0.058$$. Using the sensitivity and specificity values that we outlined above, we can compute the individual’s likelihood of having cancer given a positive test:

$P(\text{cancer|test}) = \frac{P(\text{test|cancer})*P(\text{cancer})}{P(\text{test|cancer})*P(\text{cancer}) + P(\text{test|}\neg\text{cancer})*P(\neg\text{cancer})}$ $= \frac{0.8*0.058}{0.8*0.058 +0.3*0.942 } = 0.14$ That’s pretty small – do you find that surprising? Many people do, and in fact there is a substantial psychological literature showing that people systematically neglect base rates (i.e. overall prevalence) in their judgments.

## 3.8 Learning from data

Another way to think of Bayes’ rule is as a way to update our beliefs on the basis of data – that is, learning about the world using data. Let’s look at Bayes’ rule again:

$P(B|A) = \frac{P(A|B)*P(B)}{P(A)}$

The different parts of Bayes’ rule have specific names, that relate to their role in using Bayes rule to update our beliefs. We start out with an initial guess about the probability of B ($$P(B)$$), which we refer to as the prior probability. In the PSA example we used the base rate for the prior, since it was our best guess before we knew the test result. We then collect some data, which in our example was the test result. The degree to which the data A are consistent with outcome B is given by $$P(A|B)$$, which we refer to as the likelihood. You can think of this as how likely the data are, given the particular hypothesis being tested. In our example, the hypothesis being tested was whether the individual had cancer, and the likelihood was based on our knowledge about the specficity of the test. The denominator ($$P(A)$$) is referred to as the marginal likelihood, because it expresses the overall likelihood of the data, averaged across all of the possible values of A (which in our example were the positive and negative test results). The outcome to the left ($$P(B|A)$$) is referred to as the posterior - because it’s what comes out the back end of the computation.

There is a another way of writing Bayes rule that makes this a bit clearer:

$P(B|A) = \frac{P(A|B)}{P(A)}*P(B)$

The part on the left ($$\frac{P(A|B)}{P(A)}$$) tells us how much more or less likely the data A are given B, relative to the overall (marginal) likelihood of the data, while the part on the right side ($$P(B)$$) tells us how likely we think B is (before we know anything about the data). This makes it clearer that the role of Bayes theorem is to update our prior knowledge based on the degree to which the data are more likely given B than they would be overall.

## 3.9 Odds and odds ratios

The result in the last section showed that the likelihood that the individual has cancer based on a positive PSA test result is still fairly low, even though it’s more than twice as big as it was before we knew the test result. We would often like to quantify the relation between probabilities more directly, which we can do by converting them into odds which express the relative likelihood of something happening or not:
$\text{odds of A} = \frac{P(A)}{P(\neg A)}$

In our PSA example, the odds of having cancer (given the positive test) are:

$\text{odds of cancer} = \frac{P(\text{cancer})}{P(\neg \text{cancer})} =\frac{0.14}{1 - 0.14} = 0.16$

This tells us that the that the odds are fairly low of having cancer, even though the test was positive. For comparison, the odds of rolling a 6 in a single dice throw are:

$\text{odds of 6} = \frac{1}{5} = 0.2$

As an aside, this is a reason why many medical researchers have become increasingly wary of the use of widespread screening tests for relatively uncommon conditions; most positive results will turn out to be false positives.

We can also use odds to compare different probabilities, by computing what is called an odds ratio - which is exactly what it sounds like. For example, let’s say that we want to know how much the positive test increases the individual’s odds of having cancer. We can first compute the prior odds – that is, the odds before we knew that the person had tested positively. These are computed using the base rate:

$\text{prior odds} = \frac{P(\text{cancer})}{P(\neg \text{cancer})} =\frac{0.058}{1 - 0.058} = 0.061$

We can then compare these with the posterior odds, which are computed using the posterior probability:

$\text{odds ratio} = \frac{\text{posterior odds}}{\text{prior odds}} = \frac{0.16}{0.061} = 2.62$

This tells us that the odds of having cancer are increased by 2.62 given the positive test result.

## 3.10 What do probabilities mean?

It might strike you that it is a bit odd to talk about the odds of a person having cancer depending on a test result; after all, the person either has cancer or they don’t. Historically, there have been two different ways that probabilities have been interpreted. The first (known as the frequentist interpretation) interprets probabilities in terms of long-run frequencies. For example, in the case of a coin flip, it would reflect the relative frequencies of heads in the long run after a large number of flips. While this interpretation might make sense for events that can be repeated many times like a coin flip, it makes less sense for events that will only happen once, like an individual person’s life or a particular presidential election; and as the economist John Maynard Keynes famously said, “In the long run, we are all dead.”

The other interpretation of probablities (known as the Bayesian interpretation) is as a degree of belief in a particular proposition. If I were to ask you “How likely is it that the US will return to the moon by 2026”, you can provide an answer to this question based on your knowledge and beliefs, even though there are no relevant frequencies to compute a frequentist probability. One way that we often frame subjective probabilities is in terms of one’s willingness to accept a particular gamble. For example, if you think that the probability of the US landing on the moon by 2026 is 0.1 (i.e. odds of 9 to 1), then that means that you should be willing to accept a gamble that would pay off with anything more than 9 to 1 odds if the event occurs.

As we will see, these two different definitions of probability are very relevant to the two different ways that statisticians think about testing statistical hypotheses, which we will encounter in later chapters.