Two Mistakes Hiding Behind One Good-Looking Number (opens in new tab)

I'm starting an Electronics and Communication Engineering degree this year, and a few weeks before classes began I decided to build something real instead of waiting for a syllabus to tell me what to learn: a model that detects abnormal heartbeats from raw ECG signal, small enough to run on a microcontroller, not a cloud GPU. The first version of this project hit 98% accuracy. That number was almost meaningless, and it took me two separate rounds of being wrong to find out why. The number tha...