Tracking error

Tracking error measures how closely a portfolio or fund follows its benchmark index, calculated as the standard deviation of the difference between the portfolio's returns and the benchmark's returns over a given period. A low tracking error means the portfolio moves almost in lockstep with its benchmark; a high tracking error means its returns diverge more, whether the fund beats or lags the benchmark in any given period.

It is calculated as σ(Rp - Rb), the standard deviation of the return differences, and is usually annualized. This is distinct from simple over- or under-performance versus a benchmark, which only captures the average gap; tracking error captures how variable that gap is period to period, so two funds can have identical average excess return but very different tracking errors.

Index funds and ETFs aim for a low tracking error as a sign of how faithfully they replicate their benchmark, net of the fund's expense ratio and trading costs, while active managers accept higher tracking error in exchange for the chance of outperformance, often measured alongside alpha. A common pitfall is confusing tracking error with tracking difference, the simple gap in cumulative return between fund and benchmark; the two can move in opposite directions, since a fund can have small average tracking difference but large period-to-period tracking error.