We sometimes speak of data as if data could talk. For example, we say such things as “What do the data say?” and “Let the data speak for themselves.” It turns out there’s a way to take this figure of speech seriously: **Evidence can be meaningfully measured in ****decibels**.

In acoustics, the intensity of a sound in decibels is given by

10 log

_{10}(P_{1}/P_{0})

where P_{1} is the power of the sound and P_{0 }is a reference value, the power in a sound at the threshold of human hearing.

In Bayesian statistics, the level of evidence in favor of a hypothesis H_{1} compared to a null hypothesis H_{0} can be measured in the same way as sound intensity if we take P_{0 }and P_{1 }to be the posterior probabilities of hypotheses H_{0 }and H_{1 }respectively.

Measuring statistical evidence in decibels provides a visceral interpretation. Psychologists have found that human perception of stimulus intensity in general is logarithmic. And while natural logarithms are more mathematically convenient, logarithms base 10 are easier to interpret.

A 50-50 toss-up corresponds to 0 dB of evidence. Belief corresponds to positive decibels, disbelief to negative decibels. If an experiment shows H_{1 }to be 100 times more likely than H_{0 }then the experiment increased the evidence in favor of H_{1 }by 20 dB.

A normal conversation is about 60 acoustic dB. Sixty dB of evidence corresponds to million to one odds. A train whistle at 500 feet produces 90 acoustic dB. Ninety dB of evidence corresponds to billion to one odds, data speaking loudly indeed.

To read more about evidence in decibels, see Chapter 4 of Probability Theory: The Logic of Science.

It’s a shame this doesn’t seem to be more widely embedded. I don’t care about how likely the data are under some bogus null; I care how likely they are to indicate a departure from accepted models. I think that most non-statisticians expect this as well, rather than the opaque and noisy inference generated by tests with p-values.

Relevant: http://twitter.com/mdreid/status/25410699681337345#

Working with log-odds can also be convenient in software. You can sum log-odds without worrying about underflow, whereas taking the product of lots of small probabilities will quickly get you into problems.