Open your phone and look at three numbers. A weather app saying tomorrow will be 34 degrees. A property site showing what a house in your area is roughly worth. A ride app quoting your fare before the driver has even arrived.

Every number like that comes from a model that learned from past data. Real products use far more sophisticated models than what I am about to show you. But the simplest, oldest member of that family is called linear regression, and almost everything fancier is built on the same core move. Understand that move once, in plain words, and you will start noticing it behind every prediction you see.

I teach undergraduate mathematics, and I wrote this the way I would explain it to a friend who has never opened a statistics book. If you already write code, stay for the last third, there is a short Python section for you.

You Already Do This, You Just Never Named It

Think about the last time you sold an old phone on OLX or eBay. You did not open a calculator.…