# baycomp¶

Baycomp is a library for Bayesian comparison of classifiers. (For those who don’t know what they do and how, and why you should use them instead of testing null hypotheses, we prepared a short introduction for dummies).

Functions in the library compare two classifiers on one or on multiple data sets. They compute three probabilities: the probability that the first classifier has higher scores than the second, the probability that differences are within the region of practical equivalence (rope), or that the second classifier has higher scores. We will refer to this probabilities as p_left, p_rope and p_right. If the argument rope is omitted (or set to zero), functions return only p_left and p_right.

The region of practical equivalence (rope) is specified by the caller and should correspond to what is “equivalent” in practice; for instance, classification accuracies that differ by less than 1 % may be called equivalent. So we can set a rope of 1 (if accuracies are on scale from 0 to 100, or to 0.01, if they are on a scale from 0 to 1).

Similarly, whether higher scores are better or worse depends upon the type of the score.

The library can also plot the posterior distributions.

The library can be used in three ways.

1. Two shortcut functions can be used for comparison on single and on multiple data sets. If nbc and j48 contain a list of average classification accuracies of naive Bayesian classifier and J48 on a collection of data sets, we can call

>>> two_on_multiple(nbc, j48, rope=1)
(0.23124, 0.00666, 0.7621)


(Actual outputs may differ due to Monte Carlo sampling.)

With some additional arguments, the function can also plot the posterior distribution from which these probabilities came.

2. Tests are packed into test classes. The above call is equivalent to

>>> SignedRankTest.probs(nbc, j48, rope=1)
(0.23124, 0.00666, 0.7621)


and to get a plot, we call

>>> SignedRankTest.plot(nbc, j48, rope=1, names=("nbc", "j48"))


To switch to another test, use another class:

>>> SignTest.probs(nbc, j48, rope=1)
(0.26508, 0.13274, 0.60218)

3. Finally, we can construct and query sampled posterior distributions.

>>> posterior = SignedRankTest(nbc, j48, rope=1)
>>> posterior.probs()
(0.23124, 0.00666, 0.7621)
>>> posterior.plot(names=("nbc", "j48"))


Detailed documentation is given on the following pages.