What Is Tracking Error in ETFs?

The Complete Answer

Tracking error measures how closely an index ETF follows the index it is meant to mirror. If the S&P 500 returns 10.0% in a year and the ETF returns 9.95%, the 0.05% gap is the tracking difference; tracking error specifically describes how consistent that gap is over time. Lower is better.

For large, plain-vanilla index funds the number is tiny. Funds like VOO, IVV, and VTI typically track within a few hundredths of a percent because their indexes are liquid and easy to replicate. Most of the small shortfall is simply the expense ratio being deducted from returns.

Tracking error grows in harder-to-replicate corners of the market. International funds dealing with currency conversion and time-zone gaps, emerging-market funds, small-cap funds, and any fund that "samples" an index rather than holding every component all tend to drift further from their benchmark.

When you are choosing between two ETFs on the same index, check both the expense ratio and the actual record versus the benchmark. A fund with a marginally higher fee but tighter tracking can deliver more of the index's return than a cheaper fund that consistently lags it.