# How good is the Beta distribution as a conjugate for Binomial distribution?

I understand that the Beta Distribution is a 'natural conjugate' of the Binomial distribution, in sense that the Posterior Distribution is proportional to the multiplication of both.

$$Posterior(\theta | X) \propto Likelihood(X|\theta) \cdot Prior(\theta)$$

$$\pi(\theta | X) \propto P(X=x | \theta) \cdot \pi(\theta)$$

$$\pi(\theta | X) \propto \big{[} \binom{n}{x} \theta^x (1-\theta)^{n-x} \big{]} \cdot \big{[} \theta^{\alpha-1} (1-\theta)^{\beta-1} \big{]}$$

$$\theta | X \sim \text{Beta}(x + \alpha, n - x + \beta)$$

with $$x$$ the number of successes in $$n$$ independent Bernoulli trials with success probability $$\theta$$. $$\alpha$$ and $$\beta$$ prior parameters of the beta distribution.

But I have also seen some people scaling up/down the impact of each unit of information into the posterior distribution:

$$\theta | X \sim \text{Beta}(c \cdot x + \alpha, c \cdot (n - x) + \beta)$$

In that sense, the posterior distribution can adjust the strength of the evidence provided by the data by scaling the successes and failures (With a big $$c$$ increasing the effect of the data, small $$c$$ decreasing the effect of the data).

The fact that there is an (arbitrary?) $$c$$ scaling up and down the posterior distribution makes me think that the Beta distribution, even though convenient because of the conjugacy, may not be a good representation of the distribution of $$\theta$$, otherwise, why tuning it? Is there something I am missing/ misinterpreting?

Just for a visual representation: let's define $$\alpha = 1$$, $$\beta = 1$$, $$n = 16$$, $$x = 6$$

if $$c = 0.5$$:

if $$c = 1$$

if $$c = 3$$:

• This is something I've often wondered about. But notice that what's changing with the scaling factor $c$ is not the prior, but the likelihood. Commented Feb 13 at 16:15
• Your comment made me notice that the transformation looks like $\pi(\theta | X) \propto (L(X|\theta))^c \cdot (P(\theta))$. I also researched a bit, and in the more general case Multinomial-Dirichlet, it can also be expressed as the Likelihood to the power $c$, but I don't know if it is just a nice coincidence, or if it has indeed a meaning. Commented Feb 15 at 9:39
• We (i.e. the stats research community) are also still trying to determine the exact meaning of such a transformation. For more info check out "power priors". Commented Feb 15 at 21:13

The fact that there is an (arbitrary?) $$c$$ scaling up and down the posterior distribution makes me think that the Beta distribution ... may not be a good representation of the distribution of $$\theta$$, otherwise, why tuning it?

Why not tuning it?

It's intrinsic to the Bayesian approach to be subjective. There is no unique choice for a prior. The computation of the expression $$P(\theta|x)$$ requires a prior $$P(\theta)$$, and that prior will always be prior information outside of the experiment observations. Whether it stems from a tunable distribution family or from a fixed distribution.

Even when you have according to some standard a unique prior that can not be tuned, then it is a subjective choice to use that standard.

If anything, then tuning makes the prior more practical (and better). For example:

• Starting with the Jeffreys prior, $$\propto \theta^{1/2} (1-\theta)^{1/2}$$, you obtain after an experiment a posterior $$\propto \theta^{1/2+x} (1-\theta)^{1/2+n-x}$$, and that posterior can serve as a prior for a new experiment. You don't have to use the Jeffreys prior all the time.

• In the analysis of the covid vaccine, where a conservative prior was chosen (on with prior knowledge/believe opposite to what one wants to prove with the experiment) described in this question Which statistical model is being used in the Pfizer study design for vaccine efficacy?

But I have also seen some people scaling up/down the impact of each unit of information into the posterior distribution:

$$\theta | X \sim \text{Beta}(c \cdot x + \alpha, c \cdot (n - x) + \beta)$$

This isn't exactly the tuning of the prior, and is more like an extension to the Bayesian analysis as a whole (diverging from it, it isn't the same anymore). The $$c$$ parameter is changing the likelihood function, and not the prior. This additional tuning paramter $$c$$ is a trick outside of the Bayesian framework.

What distribution/likelihood is this exactly? It needs to be a function of a form that can be partitioned like $$f(x|\theta) = g(x) h(x|\theta)$$ where $$h(x|\theta) = \theta^{cx}(1-\theta)^{c(n-x)}$$

We might see it as an exponential dispersion family. For the binomial distribution this has been described in another question: What is the dispersion parameter of binomial distribution?).

If we use that likelihood function, then the form of the dispersed binomial distribution becomes

$$f(x|\theta,c) = h(x,c) \exp\left(\frac{\theta x + A(\theta)}{1/c} \right)$$

with

$$\begin{array}{} h(x,c) &=& {n \choose x} \\ \theta &=& \log(p/(1-p)) \\ A(\theta) &=& n \log(1+\exp(\theta)) \end{array}$$

This is not a true distribution (and we can not correct it by normalizing it, which would change $$A(\theta)$$, and the likelihood).

The likelihood function with that parameter $$c$$ is a quasi likelihood function. The parameter $$c$$ can be considered as a pragmatic approach to tuning the dispersion of the binomial distribution.

• Thank you very much for your comment. Very complete. I realized that this whole thing on the Likelihood tuning is itself a rabbit hole, and I will have to dig in it. Commented Feb 15 at 9:54

I don't have my textbook in front of me, but I recall the Bayes estimator with conjugate prior is Bayes optimal meaning the posterior mode minimizes the MSE. In other words, it is a biased estimator but has much smaller variance than the MLE even when the MLE is unbiased. You can therefore generalize the Bayes to either attenuate or accentuate the impact of the data relative to the prior as a kind of shrinkage estimator which can be tuned for less bias but more variance, or vice versa.

• Ohhh, that sounds interesting. Specially since sometimes the mode of the bayesian posterior coincides with the MLE. Tbh, is the first time I hear that thing about the bias and variance tradeoff in bayesian estimation. If you could provide the reference I would thank you a lot! Commented Feb 13 at 18:02
• The Bayes estimates of $p$ when $X\sim\mathcal B(n,p)$ are admissible but they do not "minimize the MSE" since they are not comparable uniformly over $p$. The MLE is a generalised Bayes estimate that is also admissible in that case for $n\le 5$ and inadmissible for $n\ge 6$. However, this is not obvious for the attenuated version, which is not a Bayes estimator and hence cannot claim automatic admissibility. Commented Feb 13 at 21:22
• ps-The Binomial case is covered in Chapter 2 of my book. Commented Feb 13 at 21:27
• @OscarFlores Xi'an is far more adept at this than I, however a slide deck which clarifies my answer in much better terms can be found here: pages.stat.wisc.edu/~shao/stat610/stat610-02.pdf Commented Feb 13 at 22:31
• Thank you both for your comments. Can I then assume that for n <= 5, there's no need of tuning the Likelihood, but for n >= 6, it can be helpful? (or how should I interpret the admissibility in relation to the scaling factor $c$?) Commented Feb 15 at 10:00