The transaction code is the whole story. A P for an open-market purchase is the informative one, because an insider spending personal money is a discretionary act. An S following an M or an A is usually an option exercise or vesting event, and much of that selling runs on a pre-arranged plan adopted months earlier.
Aggregate cluster buying by several insiders at once has historically been more predictive than any single transaction, and sales are noisy because insiders diversify for reasons unrelated to the outlook.
Example: three directors and the CFO file Form 4s within a week reporting open-market purchases totalling $2.4M at an average $11.80, following a fall from $19. That cluster is a different signal from one scheduled sale.
Related: insider-trading, form-144, schedule-13d, stock-based-compensation, employee-stock-options