The number of applications of Bayesian inference has been growing rapidly and will probably continue so over the next 20 years. For example, it is now widely used in astrophysics. Certain theories of cosmology contain fundamental parameters—the curvature of space, density of visible matter, density of dark matter, and dark energy—that are constrained by experiments. Bayesian inference can pin down these quantities in several different ways. If you subscribe to a particular model, you can work out the most likely parameter values given your prior belief. If you are not sure which model to believe, Bayes’s rule allows you to compute odds ratios on which one is more likely. Finally, if you don’t think the evidence is conclusive for any one model, you can average the probability distributions over all the candidate models and estimate the parameters that way.
Bayesian inference is also becoming popular in biology. For instance, genes in a cell interact in complicated networks called pathways. Using microarrays, biologists can see which pathways are active in a breast cancer cell. Many pathways are known already, but the databases are far from perfect. Bayesian inference gives biologists a way to move from prior hypotheses (this set of genes is likely to work together) to posterior ones (that set of genes is likely to be involved in breast cancer).
In economics, a Bayesian analysis of consumer surveys may allow companies to better predict the response to a new product offering. Bayesian methods can burrow into survey data and figure out what makes customers different (for instance, some like anchovies on their pizza, while others hate them).
Bayesian inference has proved to be effective in machine learning—for example, to teach spam filters to recognize junk e-mail. The probability distribution of all e-mail messages is so vast as to be unknowable; yet Bayesian inference can take the filter automatically from a prior state of not knowing anything about spam, to a posterior state where it recognizes that a message about “V1agra” is very likely to be spam.
While Bayesian inference has a variety of real-world applications, many of the advances in Bayesian statistics have depended and will depend on research that is not application-specific. Markov chain Monte Carlo, for example, arose out of a completely different field of science. One important area of research is the old problem of prior distributions. In many cases there is a unique prior distribution that allows an experimenter to avoid making an initial estimate of the values of the parameters that enter into a statistical model, while making full use of his or her knowledge of the geometry of the parameter space. For example, an experimenter might know that a parameter will be negative without knowing anything about the specific value of the parameter.
Basic research in these areas will complement the application-specific research on problems like finding breast cancer genes or building robots and will therefore ensure that Bayesian inference continues to find a wealth of new applications.
Bayesian inference is also becoming popular in biology. For instance, genes in a cell interact in complicated networks called pathways. Using microarrays, biologists can see which pathways are active in a breast cancer cell.