K-Fold Cross-Validation: The Comedian Who Tested Jokes at Only One Comedy Club and Bombed Everywhere Else

You tested your model on one train/test split. Accuracy was 94%! You deployed it. Real-world accuracy was 81%. What happened? You got lucky (or unlucky) with your single split. K-fold cross-validation tests on K different splits and averages — so you know what to ACTUALLY expect. Here's how it works and how to choose K.