How an AI learns · part 8 of 11 · Training set and test set: shuffle, 80/20, how much to hide

If you saw the exam yesterday, what does 100% prove?

An exam stamped 100 beside yesterday's same paper, a spam filter that scores 100% on old emails but about 70% on new ones, and Google Flu Trends' real 2012–13 miss show why we lock a test set away. Then a year of weather is shuffled before an 80/20 cut, and a 1% slice of a million fills a stadium. Turn the share hidden for testing and watch the knife move.

What you turn: Share hidden for testing

The reel