Statistical Thinking

Explaining p-values with puppies

You’ll find p-values lurking all over data science (and all the rest of science, for that matter). If you took STAT101, the explanation you probably heard runs something like this: A p-value is the probability of observing a statistic at least as extreme as ours, conditional on the null hypothesis. No wonder that didn’t stick! Let’s try it with puppies instead…

(If you prefer to learn in video form, link’s at the bottom. Pure audio? Here.)

Is p-value short for puppy-value?

Setting the (crime) scene

Imagine coming home and discovering this in your kitchen:

Let’s assume this is your dog and your kitchen, otherwise the example just became much stranger. Also, as far as their owners are concerned, dogs are always puppies even when they’re too big to carry around.

Let’s put this suspect on trial for the crime of sticking his head in the garbage bin!

We’ll work with a default action of not yelling at Fido and a corresponding null hypothesis of “Fido is innocent.” If you’re new to these concepts or unsure how to set up hypotheses, read this.

Describe the null world

The first step in calculating a p-value is to take a deep breath and say, “Okay, Fido, I’m going to imagine that you are innocent.”