What an edge is

An edge is a reason your strategy should make money that isn’t luck. It’s usually small, and it always has to survive the costs of trading.

The useful way to think about it isn’t “how often am I right”, it’s expectancy — what one average trade is worth.

expectancy = (win rate × average win) − (loss rate × average loss)

Take a strategy that wins 40% of the time, making $300 on winners and losing $150 on losers:

(0.40 × $300) − (0.60 × $150) = $120 − $90 = +$30 per trade

Positive. Worth running, even though it’s wrong most of the time. Now subtract $8 a trade in spread and slippage and it’s +$22. Still fine — but you can see how a thinner edge disappears.

Why most ideas don’t have one

Because the obvious ones are already taken. If “buy stocks that went up yesterday” reliably worked, enough people would do it that the price would move before you got there. Markets aren’t perfect, but they’re efficient enough that free money doesn’t sit around.

Real edges tend to come from one of a few places:

• Doing something inconvenient. Holding through discomfort other people won’t.
• Being slower, not faster. Competing with professional firms on speed is not a fight a retail trader wins.
• Being consistent. Following a mediocre rule every single time beats following a good rule when you feel like it.
• Risk management. Not losing badly is an edge, and it’s the most reliably available one.

How to tell if you have one

You mostly can’t, quickly. That’s the honest answer, and it’s why so much of this library is about testing carefully rather than trading confidently.

What you can do is rule things out. A strategy with negative expectancy in a backtest almost certainly won’t have positive expectancy live. A strategy whose edge depends on trading 200 times a year is probably paying it all away in costs. And a strategy that only works with nine tuned parameters has probably fitted the past rather than found anything.

What’s left after all that ruling-out is where to spend your attention.