Long Format

Long format stores data with many rows and few columns: typically one row per observation-per-measurement, with columns like id, variable name, and value. A student's three test scores become three rows, each naming the test and its score.

Long format is what most plotting and statistical tools want, because the thing you want to color or group by lives in its own column. It can look repetitive to human eyes, but it scales cleanly when you add new measurement types — you add rows, not new columns.