Free financial data can support serious backtesting—but only if the source matches the question.

A price-momentum test needs historical prices and corporate-action handling. A macro strategy needs revision-aware economic series. A fundamental strategy needs values as investors actually knew them on each historical date. Those are different data problems.

This guide compares SEC EDGAR, yfinance, Stooq, FRED, Alpha Vantage’s free tier, and Tradevo Data’s public proof pack. The goal is not to declare one winner. It is to show where each source may be useful, where it can fail, and whether it is point-in-time safe for the intended test.

The short answer

SEC EDGAR: a free primary source for US filings, but substantial engineering is required to produce clean point-in-time panels.

yfinance: convenient for exploration and price-data prototypes, but do not assume every historical or fundamental field is point-in-time safe.

Stooq: potentially useful for accessible market-history research; validate adjustments,…