Turnover

What it is

Turnover is how much of a portfolio is replaced per period. High turnover means the strategy trades often.

Why it is the cost multiplier

Costs scale with turnover. A rule that looks profitable before costs can be unprofitable after them, purely because it trades too much.

Turnover	What it does to a backtest
Low	Costs barely register — the result is robust
High	Costs dominate — and a small slippage error is fatal

High turnover is where the slippage assumption stops being a detail and becomes the result.

How to check

Ask for the turnover number and the cost model together. Turnover without costs is a strategy with the bill removed.