What is backtesting, and how is it done honestly?

Backtesting replays a rule on the past

Backtesting runs a strategy or model over historical data to see how it would have behaved. Done carefully it is a useful sanity check. Done carelessly it becomes a machine for manufacturing false confidence, because it is easy to accidentally use future knowledge or ignore real world frictions.

Costs, slippage, and capacity

A realistic backtest includes trading costs, the spread, slippage when orders move the price, and the limit on how much size a strategy can actually absorb. Many impressive backtests collapse the moment these real costs are added, which is exactly why they must be included from the start.

Out-of-sample and walk-forward testing

The strongest practice is to reserve data the model never saw, and to test in a walk forward manner that mimics how a model would actually be retrained over time. Even then, a good backtest is a necessary condition, never a promise. Live results are routinely worse.

This article is general educational information only. It is not financial, investment, or trading advice, and not a recommendation to buy or sell any instrument. Markets carry risk of loss, and past performance does not guarantee future results. Consult a qualified, licensed professional before making any decision.