Bayes-Ball

Suppose you are a longtime Red Sox fan and you just bought a ticket to the fourth of a four-game series against the Yankees. The Red Sox have won two of the three previous games in the series. As you settle into your seat at Fenway, everyone around you is confident the Red Sox will win. At least that’s the best bet. But what are the odds on that bet? Even if you are loathe to bet on the Yankees, what is the chance the Red Sox lose?

The Red Sox have won two out of the three previous games, so it is reasonable to think that the probability that they win the next game is $2/3 \approx 66.6$ percent. But maybe the Red Sox’ win probability against the Yankees is actually higher, say $75$ percent, and they just got unlucky in the first game. Or maybe the Red Sox’ win probability is actually lower—$50$ or even $40$ percent—and they just happened to be on a hot streak. Guessing $66.6$ percent seems reasonable, but how do we know this is even a good estimation scheme?

Suppose we only watched the first game, which the Red Sox lost. If we used the same estimation scheme, then we’d be forced to say that the Red Sox have a $0/1 = 0$ percent win chance in any future game. This is clearly not a reasonable guess since there has to be some chance that the Red Sox win. One game does not provide enough information to make this estimator reasonable. Why? Because we’ve seen the Red Sox win other games and we know even the best MLB teams don’t have a $100$ percent win probability against the worst teams.

This statement—no team has a $100$ percent win probability against any other team—is an example of prior knowledge. We know something about baseball before we see any games. This prior knowledge lets us reject simple estimation schemes when they give answers that are wildly off-base. Can we use prior knowledge to improve our estimator so that it doesn’t go foul?

Questions

The story above raises three main questions:

1. How should we estimate the win probability of team 1 against team 2 if we see team 1 win $w$ out of $n$ games against team 2?
2. How can we incorporate prior information about baseball to improve this estimate?
3. What prior information do we actually have about win probabilities in baseball?

The answers to all three of these questions depend on one number, $\beta$. The value of $\beta$ represents our prior knowledge about baseball. It is large when teams are often evenly matched (win probabilities near $50$ percent) and small when teams are often unevenly matched (win probabilities near $0$ or $100$ percent). A large $\beta$ means most games are unpredictable; a small $\beta$ means most games are predictable.

Given $\beta$, there is a surprisingly simple way to estimate win probabilities that accounts for prior knowledge. This method can be motivated rigorously while remaining easy to compute. It is so easy to compute that you could use it while sitting in the bleachers with Cracker Jacks in one hand and a hot dog in the other.

But to use this method you need to know $\beta$. In this article, we fit for $\beta$ based on the history of baseball. By estimating $\beta$, we gain insight into the nature of the sport and how that nature has changed over the league’s 148-year history.

How to Incorporate Prior Information

Before we get to $\beta$, we need to understand the question we are asking. Specifically, how to balance prior knowledge with observed games and arrive at a probability.

To do this we need some notation. Capital letters are used for quantities that are either random or unknown while lower case letters are used for quantities that are either fixed or known.

Let $n$ be the number of games observed and $w$ be the number of games team 1 won. Since there is a chance that either team could win or lose any game, observing $w$ wins is a specific outcome out of many possible outcomes. Let $W$ be a random variable representing the number of wins if we could run the $n$ games again. This is analogous to seeing a coin land heads $w$ times out of $n$ flips. $W$ is the number of times it lands heads if we flip $n$ more times. Let $P$ be the probability that team 1 beats team 2. Assume that this probability is independent of the outcome of all previous games and is constant over the course of the $n$ games.

In order to estimate $P$, we want to know the likelihood that $P = p$ given that $W = w$ for any possible outcome $w$. We could then find what win probability is most likely given the observed outcome. The likelihood that $P = p$ given $W = w$ is the conditional probability: $\text{Pr}\{P = p|W = w,n \}$.

To find this conditional probability, we will use Bayes’ rule.

Bayes’ rule expresses the probability that a random event $A$ occurs given that $B$ occurred in terms of the odds that $B$ occurs given that $A$ occurred. It reverses the direction of the conditioning. This reversal is useful because conditional probabilities are often easier to work out in one direction than in the other. Bayes’ rule comes from the following pair of equalities:

Setting the two equal to each other and solving for $\text{Pr}\{A|B\}$ yields:

Applying Bayes’ rule to our problem gives:

The conditional probability on the left hand side, $\text{Pr}\{P = p|W = w,n\}$, is the posterior. This is the probability that the $P = p$ given that team 1 won $w$ out of $n$ games. It is called the posterior because it is the distribution of win probabilities after observing data. Our goal is to find the probability $p$ that maximizes the posterior.

On the right hand side, $\text{Pr}\{W = w|P = p,n\}$, is the likelihood, so named because it is the likelihood of observing the data given a win probability $p$. The probability $\text{Pr}\{P = p\}$ is the prior. This is the probability that team 1 has a win probability $p$ against team 2 before we observe any games between them. We’ve done it! This is how we incorporate prior knowledge.

The Prior

Now that we know how to incorporate prior knowledge, what do we want it to be? What distribution should we use to model the probability that a baseball team has win probability $P = p$ against another team?

We will use a symmetric beta distribution since the beta distribution is the standard choice for this type of estimation problem (don’t worry, the reasons will become apparent once we start the analysis). This is where $\beta$ enters; it is the parameter that defines this distribution.

When $\beta = 0$, the distribution is uniform—all win probabilities are equally likely. This is the same as knowing nothing about the win probabilities. When $\beta = 1$, the distribution is a downward-facing parabola. When $\beta > 1$, the distribution is bell-shaped. The larger $\beta$, the more the distribution concentrates about $p = 1/2$. In essence, a large $\beta$ means less predictable games and a more competitive league. You can experiment with different $\beta$s below.

The more concentrated the distribution, the less likely it is that team 1 has a large (or small) win probability against team 2. Therefore, by tuning $\beta$ we can express our expectation about how even baseball teams are. We will use past baseball data to fit for this parameter. Note that this prior does not incorporate prior information about specific teams, only about the nature of baseball as a whole.

The Estimator

Now we can state our original question formally. If team 1 wins $w$ out of $n$ games against team 2, and if the win probabilities are sampled from a beta distribution with parameter $\beta$, then what is the most likely win probability for team 1?

This may seem like a hard question to answer cleanly—and in general it is. However, by picking the beta distribution as our prior, the answer becomes both elegant and intuitive.

The most likely win probability $p_*$, having seen $w$ wins out of $n$ games, given prior parameter $\beta$, is:

This win probability maximizes $\text{Pr}\{P = p|W = w,n\}$. That’s all there is to it!

What makes this so brilliant is that the parameter $\beta$ can be treated as a number of fictitious games. Think back to our original method for estimating the win probability: the win frequency (ratio of wins to games played). The most likely win probability is just the win frequency if we pretend that our team wins $\beta$ more games and loses $\beta$ more games.

For example, suppose that $\beta = 1$, $w = 2$, and $n = 3$. Instead of estimating that the Red Sox have a $2/3$ win probability, we estimate that they have win probability $(2 + 1)/(3 + 2) = 3/5$, just as if they had won and lost one more game. If instead $\beta = 4$, then our estimated win probability would be the Red Sox’ win frequency had they won an additional 4 games and lost an additional 4 games: $(2 + 4)/(3 + 8) = 6/11$.

Notice that as $\beta$ grows, the estimated win probability approaches $1/2$. As a consequence, if $\beta$ is large, then our estimator is conservative, and it will take a lot of games to move the estimated win probability far away from $1/2$.

You can experiment with different $w$, $n$, and $\beta$s below. While you experiment, consider how large you think $\beta$ should be for baseball.

The proof that $p_*(w,n|\beta)$ maximizes the posterior is included below (in two pieces). The proof is not necessary to understand the role of $\beta$ in the estimator; however, it is important to understand why the estimator takes this form. In particular, the proof below is needed to understand exactly what is assumed a priori about baseball win probabilities given a particular $\beta$. The motivation for the beta prior is explained in the second half of the proof. That said, if you are only interested in how $\beta$ has changed over time and how $\beta$ differs between different sports, skip to the end of the proof and continue from there.

Proof Without Prior ($\beta = 0$)

Why does the most likely win probability take this form?

If we do not incorporate prior information, then the posterior is proportional to the likelihood and maximizing the posterior is equivalent to maximizing the likelihood. To find the likelihood, condition on $P = p$ (assume that the win probability is $p$). Then the number of wins $W$ is binomially distributed, which means that:

So in the absence of prior information:

Note that $P$ is not binomially distributed because the posterior is a distribution over $p$ not $w$. When using the expression on the right as a posterior it is important to remember that $p$ is the variable that can change and $w$ and $n$ are fixed by what happened in the observed games.

This distribution is an example of a beta distribution. A beta distribution is a probability distribution for probabilities. It is often used to give the probability $P = p$. The beta distribution depends on two parameters, $\alpha, \beta$. It has the form: ¹

where:

and $\Gamma(x)$ is the gamma function. The gamma function is the continuous extension of the factorial. That is: $\Gamma(n+1) = n!$ for any integer $n$, but, unlike the factorial, $\Gamma(x)$ is defined for all real $x$. It follows that if $\alpha$ and $\beta$ are integers, then $1/B(\alpha,\beta)$ equals $\alpha + \beta$ choose $\alpha$. Therefore, our likelihood is beta-distributed with parameters equal to the number of wins, $w$, and losses, $w-n$:

The likelihood allows us to calculate the probability that $P = p$ having observed $w$ wins out of $n$ games (in the absence of prior information). What win probability, $p_*$, maximizes the likelihood?

To maximize the likelihood we maximize the log-likelihood. Since a logarithm is monotonically increasing in its argument the log-likelihood is maximized wherever the likelihood is maximized. The log-likelihood is:

Differentiating with respect to $p$:

Setting the derivative to zero requires $\frac{w}{p_*} = \frac{n-w}{1-p_*}$ or:

This is solved when $p_* = w/n$. The win frequency is the only solution to this equation, so the log-likelihood has one extremum. The log-likelihood is a convex function, so this extremum is a maximum. Thus the win probability that maximizes the likelihood is the win frequency:

This is a good motivation for our naïve estimation scheme. If the Red Sox win two out of three games then we guess that their win probability is $2/3$. This, however, is the solution without any prior information. It is equivalent to the solution when we assume that all win probabilities are equally likely. As noted before, this solution breaks down if $w = 0$ or $n$ since it returns either a $0$ or $100$ percent chance of victory in future games. What we need is a prior distribution that can be fit to baseball data so the estimator does not return extreme win probabilities.

Proof Incorporating the Prior ($\beta \geq 0$)

In order to incorporate a prior we need to pick a form for the prior distribution, $\text{Pr}\{P = p\}$. The classic choice of prior for this problem is our home favorite, the beta distribution. The beta distribution is the standard choice of prior for this problem because it is the conjugate prior to the binomial distribution.

A conjugate prior is a distribution that, when chosen as a prior, ensures that the posterior distribution has the same form as the likelihood, only with different parameters. In our case, if we use a beta distribution as a prior, then the posterior is also beta-distributed, only with different parameter values. These parameters depend on both the data observed and the parameters of the prior.

Recall that the beta distribution depended on two parameters, $\alpha$ and $\beta$. In our case we require that our prior is symmetric about $p = 1/2$, because if team 1 has a win probability of $p$ against team 2, then team 2 has a win probability of $1 - p$ against team 1. If we did not assume the prior was symmetric in this way, then the probability a team wins against another would depend on the order we listed them in. Requiring that the beta distribution is symmetric requires $\alpha = \beta$. This means that our prior only depends on one parameter and has the form:

The product $p (1-p)$ is a downward facing parabola that is maximized at $p = 1/2$ and equals zero at $p = 0$ and $p = 1$. Changing $\beta$ bends this parabola. For $\beta > 1$ the parabola gets bent down into a bell shape. For $0 < \beta < 1$ the parabola is bent up towards a box. At $\beta = 0$ the box becomes the uniform distribution. This effect was illustrated in Demo 1.

When symmetric, the beta distribution has mean $1/2$ and variance $1/(8 \beta + 4)$. Therefore, as $\beta$ goes to infinity the variance in the beta distribution vanishes. In that case almost all of the win probabilities are expected to be close to $1/2$. This leads to the simple intuition:

1. Large $\beta$ $\rightarrow$ win probabilities near $1/2$ $\rightarrow$ unpredictable games, even teams, competitive league
2. Small $\beta$ $\rightarrow$ win probabilities can be near $0$ or $1$ $\rightarrow$ more predictable games, uneven teams, less competitive league

How does introducing this prior change our estimator?

The maximum likelihood estimator (MLE) is the probability $p$ that maximizes the likelihood. The maximum a posteriori estimator (MAP) is the probability $p$ that maximizes the posterior. Using a symmetric beta distribution with parameter $\beta$ sets the posterior proportional to:

As promised, the posterior has the same form as the likelihood, only now the parameters are $w + \beta$ and $n - w + \beta$. Therefore, by using a beta prior, the posterior distribution takes the same form as the likelihood had we seen an additional $\beta$ wins and $\beta$ losses. The parameter $\beta$ is, in effect, the number of fictitious games added to the record when accounting for prior information. Symbolically:

Since we already know how to maximize the likelihood, we also already know how to maximize the posterior. The likelihood is maximized at the win frequency so the posterior is maximized at the win frequency—after $\beta$ wins and losses have been added to the record:

$\beta$ and the Nature of Baseball

We now have an intuitive way to incorporate prior information about the win probabilities into our estimate for the win probability given data. What remains is to have an estimate for $\beta$ (now that we know how to use prior information we need to gather the prior information).

In order to estimate $\beta$ we need to:

1. gather historical baseball data to use in the estimate
2. work out an estimation framework for estimating $\beta$ given the data

FiveThirtyEight provide the score of all 220,010 MLB games played since 1871 here.³ This data can be used to estimate $\beta$ for every year of Major League Baseball. In order to smooth the results, we fit for $\beta$ on sliding ten-year intervals (i.e., we find the best fit for $\beta$ for 2010 to 2019, 2009 to 2018, and so on back to the decade spanning from 1871 to 1880). These intervals are indexed by the last year in the interval so that our estimate for $\beta$ in 2019 is based on all the years between 2010 and 2019. This way the estimate for $\beta$ in a given year does not depend on games that have not yet occurred.

To fit for $\beta$ we used a Bayesian estimation framework much like the framework derived in this article for estimating the win probability. We did not assume any prior information about $\beta$ and solved for the value of $\beta$ that maximized the likelihood of $\beta$ given the corresponding decade of baseball. As before, this likelihood was solved for by first finding the likelihood of sampling the observed decade of baseball conditioned on a given $\beta$. Then the likelihood of $\beta$ given the decade was computed using Bayes’ rule. Formally, this likelihood is the product of a series of beta-binomial distributions. Details are included in the expandable box below.

We also computed the 95 percent confidence interval on our estimate for $\beta$ using the same likelihood. If baseball win probabilities are beta-distributed, then there is a 95 percent chance that $\beta$ is in the interval if it is sampled from its posterior.

The results of this analysis are shown in Figure 3.

The average results for each decade are provided in Table 1.

Using this table we can easily compute the MAP estimate for the win probability of a baseball team that won $w$ out of $n$ games in any decade. For example, if the Red Sox beat the Yankees 2 out of 3 games in 1885 (ignoring that neither team existed at the time), then the MAP estimator for the win probability would be $(2 + 1.9)/(3 + 3.8) = 3.9/6.8 = 0.57$. Alternatively, if we saw the same thing today, the MAP estimator would be $(2 + 21.9)/(3 + 43.8) = 0.511$. Seeing the Red Sox win 2 out of 3 games only makes our estimate $1$ percent larger than $50$ percent!

Tragically, the Yankees won 14 out of 19 games against the Red Sox in 2019, which means our best estimate for the Red Sox win probability against the Yankees in 2019 is $(5 + 21.9)/(14 + 43.8) = 0.46$. This is still shockingly close to $50$ percent given that the Red Sox’ win frequency against the Yankees in 2019 was only $0.26$.

These examples illustrate how astonishingly large $\beta$ is for modern baseball. Modern baseball teams have proven to be fairly even, so we expect most team against team win probabilities to be near $0.5$. The beta distribution with $\beta = 21.9$ is shown below in Figure 4. Note that very few teams are expected to have a win probability greater than $60$ percent against any other team. This means that most matchups are relatively fair and fairly unpredictable.

The astonishing evenness of modern baseball is reflected in how large $\beta$ is. With $\beta = 21.9$ it would take winning eleven games straight to push the estimated win probability above $0.6$ and forty-four games straight to push it past $0.75$. The median number of games played between pairs of teams who competed in 2019 was seven games, and the max was nineteen, including playoffs. Few teams even play enough games to push the estimate past $60$ percent or beneath $40$ percent. Note that $\beta$ is so large that in all cases we add more fictitious games to the estimator than we do real games.

Baseball wasn’t always this even. Take a look back at Figure 3 and notice the gradual increase in $\beta$ over time. At the start of the league, $\beta$ is less than one. In fact, in 1880 the MLE estimate for $\beta$ is $0.62$ (with a confidence interval $[0.35,1.02]$). The gradual increase in $\beta$ over the history of the league reflects the league becoming more competitive. Over time teams have become more even and the playing field more level. It is now so level that it takes over twenty games—more games than are played between almost any pair of teams—before observed data has a bigger influence on our estimate than our prior expectation does.

Unlike the MLB, both the NBA and NFL have had close to constant $\beta$s over time. Both also have much smaller $\beta$s than the MLB. For example, in the 2010s the NBA and NFL have $\beta$s equal to $2.0$ and $1.7$ games respectively. This indicates that both the NFL and NBA are far more predictable than baseball, and football and basketball teams are far less evenly matched than baseball teams.

For example, if we saw the Celtics beat the Knicks 2 out of 3 games, we would estimate the Celtics win probability was $(2 + 2)(3 + 4) = 4/7 = 0.57$. The same situation in baseball would lead to a predicted win probability of $0.51$. For a more extreme example, imagine the Celtics and Red Sox each win 9 out of 10 games against the Knicks and Yankees respectively. The best estimate of the Celtics’ win probability is $(9 + 2)/(10 + 4) = 0.79$ while the best estimate for the Red Sox’ is $(9 + 21.9)/(10 + 43.8) = 0.57$. The Celtics’ win probability is only $0.11$ less than their win frequency, $0.9$, while the Red Sox’ is $0.33$ less than $0.9$.

It is also interesting to note that the likelihood distribution for $\beta$ skews much more positive for football than for basketball or baseball. This is because football teams play far fewer games per pair and much less data is available for fitting $\beta$ to any decade of NFL games. The median number of games played between any pair of NFL teams who competed with each other in 2018 was one game and the max was three. Any pair of teams who play only one game do not provide any information for estimating $\beta$, so the estimate of $\beta$ is limited to pairs of teams who play two or more games (teams within a division play twice against each possible opponent, and teams may meet more than once if they play in the regular season and in the playoffs). The uncertainty in football is most dramatic in the very early years of the league, when there were more teams and fewer games per pair, and in the 1960s, when the AFL joined the NFL (see Figure 6).

Full results for basketball and football are provided below in Table 2.

Taken together, these estimates of $\beta$ tell a story about the nature of these leagues. It is clear that baseball is an outlier when compared to the NFL and the NBA. Not only is the league much older and not only do baseball teams play far more games, but baseball teams are more evenly matched than football or basketball teams, and baseball teams have gotten more evenly matched over time. You would have to go back to 1880 (or at least 1900) for the teams in MLB to be as unevenly matched as the teams in professional football or basketball. Baseball is now so even that it is difficult to estimate teams’ win probabilities against each other without seeing close to twenty games between them. Until then, your estimates are more informed by $\beta$ than the games you watched. Until then, your estimates are more Bayes'-ball than baseball.