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An Expert Advisor (EA) is a piece of code that runs inside a trading platform (like MT4 or MT5) and executes a strategy automatically — watching for the same conditions a manual trader would, then placing, managing, and closing trades without a person clicking a button.
Because an EA follows its rules exactly and without emotion, it removes hesitation and impulse decisions from execution — but it only ever does what it was actually programmed to do, including in situations the strategy wasn't designed for, like unusual news events.
Before relying on one, most traders backtest an EA against historical data and forward-test it on a demo account, to understand not just its average results but how it behaves during its worst losing streaks — since that's the scenario that actually tests whether the risk management underneath it is sound.
Real-World Example
A surprise central bank announcement sends a currency pair gapping sharply outside its normal range, and a trader's EA, built only for calm trending conditions, keeps opening new positions exactly as programmed, unaware the market context has fundamentally changed. Since it only executes its coded rules, it can't recognize that the situation itself falls outside what it was actually designed for.
This lesson is free — no purchase needed to keep learning.