Free tick data is better than nothing, and worse than expected

[Shadow Trader]

Several sources offer free historical tick data for forex. They're widely used for backtesting scalping strategies, and they have limits worth understanding.

The data usually comes from a single liquidity source or a single broker, which means it represents one view of the market, not the market. A strategy tested on it and traded at another broker faces different prices.

Spreads in historical files may be recorded properly or may be a simplified version. Some files have bid and ask, others have bid only with an assumed spread.

Gaps in the data are common. Missing minutes, missing days, and periods where the feed froze and repeated the same price. A backtest that treats those as real market behaviour is testing a fiction.

Timestamps need checking. A file in a particular time zone, with or without daylight saving adjustments, can shift all the time-based rules by an hour.

Verification is simple but tedious: compare a few known events, such as a payrolls release, against the file and confirm that the spike appears at the right moment and the right size.

For serious work, a paid source or the broker's own data is safer, because it matches what will be traded.

Which data source has proved the most trustworthy, and what check revealed its weaknesses?