The same pairs, the same hours, two feeds. On one, the bar's open sits where a real trade happened. On the other it is manufactured from the close — and the ratio that exposes it is one line of code.
On a genuine feed, the distance between a bar's open and its close is a real fraction of that bar's range. Measure |open − close| / (high − low) across a few hundred bars and a broker feed returns a median near 0.464.
Run the identical measurement on a common free feed and it returns 0.020. The open is not being observed; it is being synthesised from the previous close. The bar looks complete, it plots correctly, and it is fiction.
This matters most to exactly the strategies people backtest first. Anything entering at the next bar's open, anything holding overnight, anything trading a gap — all of it was measured against a price nobody could have traded. The equity curve is not optimistic; it is describing a different market.
The check costs milliseconds and needs no special data: compute the median ratio on your own bars. Below about 0.10, stop and find a better source before you interpret anything else.
Run this on your own data.
Money Mind sells the check, not a strategy — we have no proven edge and do not sell signals. See the /dataquality spec (free) or browse the shop.