# "Real men keep p values to themselves"

## A short lesson in statistics

### Published January 14, 2009 7:28PM (EST)

In "The Amazing Shrinking Citigroup," posted Tuesday, I asked my readers to explain Felix Salmon's statement that "I'd say p=0.3 right now that Barack Obama's first major act as POTUS will be the nationalization of Citigroup." I confused matters by transcribing "p=.3" as "p=.03," as a number of readers immediately pointed out.

Two other regular readers responded by private e-mail, and I'm sharing those messages with you because they were elucidating, and funny.

P=.03 should mean that the probability of the event occurring is equal to the 97 percent confidence interval. Since most statistical analysis, ie that not done by methodologists, uses p < .05, i.e. the 95 percent confidence interval, as the cut off for significance, this appears to be a very specific call out of what should be considered a statistically significant event. Personally, I'd like to see the betas, bs, and standard errors so I could see just how much movement change in Y there is for every unit of change in X, as well as the directions.

"The views expressed here are mine alone and do not necessarily reflect those of the US Army's Human Terrain System, the Training and Doctrine Command, and/or the US Army".

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P -- as in "p-value" or:

The probability of what you have observed in the data, if your model of the world is true. People generally want this value to be small: a small p-value implies that the reduced model is unlikely, that your observations suggest something non-null is afoot.

People pick a cutoff -- arbitrarily, really -- and a p-value lower than the cutoff is pronounced "Significant." Alchemy!

A large p-value, alternately, is uninteresting: the data is consistent with what you already know.

Frankly -- p-values are a bit passe, if not completely gauche -- statistically speaking. Modelling, these days, is more particular (if not exact). Macho statisticians are proud of tight (posterior (bayesian)) confidence intervals. Real men keep p-values to themselves.

Kobi Ako Abayomi, Asst. Professor Georgia Institute of Technology, School of Industrial and Systems Engineering -- Statistics Group

### By Andrew Leonard

Andrew Leonard is a staff writer at Salon. On Twitter, @koxinga21.

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