Tracking Error

Tracking error is a measure of how closely an investment follows the index or benchmark it is supposed to be copying. If a fund promises to mirror, say, a stock market index, tracking error tells you how much its actual returns drifted away from that index's returns over time.

It's usually expressed as a number, often the standard deviation of the difference between the fund's returns and the benchmark's returns over a period. A tracking error of zero would mean the fund moved in perfect lockstep with the index every single day. In practice that never happens, because funds have fees, hold slightly different weightings, trade at different times than the index calculation, or hold cash for redemptions — all of which cause small daily gaps between fund performance and index performance.

The nuance that trips people up is that tracking error is not the same as underperformance. A fund can have low tracking error but still lag the index every year by a small, consistent amount (usually close to its fee), and that would show up as low tracking error because the gap is stable and predictable. Conversely, a fund could occasionally beat the index by a lot and lag it by a lot in other periods — even if the average difference is zero, that volatility of difference is what tracking error actually measures.

This term applies mainly to index funds, ETFs, and other products designed to replicate a benchmark. It matters less for actively managed strategies, where deviating from the index is the entire point rather than a flaw.