Tracking Error vs. Tracking Difference: What's the Difference?
Index funds promise to mirror a benchmark, but “mirror” is never perfect, and two different numbers are used to describe exactly how imperfect it is.
The short answer
Tracking difference measures the total return gap between a fund and its benchmark over a given period — essentially, how much the fund actually underperformed or outperformed. Tracking error measures the volatility, or consistency, of that gap over time using a statistical measure of variation. A fund can have a small tracking difference but a large tracking error, or the reverse, because one describes the size of the gap and the other describes how steady it is.
Why these are two separate questions
Imagine two funds that both end the year 0.20% behind their index. One fund got there by underperforming a steady 0.20% almost every single day. The other bounced between beating the index by 1% and trailing it by 1.4% throughout the year, and those swings happened to net out to the same 0.20% gap by December. Both funds have an identical tracking difference for the year, but very different tracking error, because the second fund’s day-to-day relationship with its benchmark was far less predictable.
What tracking difference actually captures
Tracking difference is typically expressed as an annualized percentage and answers a simple question: over this stretch of time, how much return did the fund give up (or occasionally gain) compared with its benchmark. It’s calculated after the fact by comparing cumulative returns. Because it’s a single backward-looking number, it can be affected by one-time events — a large cash inflow, a rebalancing cost, or a stretch where securities lending income happened to run high or low.
What tracking error actually captures
Tracking error is a statistical measure, usually the standard deviation of the daily or monthly differences between fund returns and benchmark returns, then annualized. It doesn’t tell you whether the fund is ahead or behind — it tells you how much the size of that gap has bounced around. A fund with low tracking error is behaving predictably relative to its index, even if it’s chronically a little behind. A fund with high tracking error is less predictable, which can matter for anyone comparing funds that are meant to serve the same role in a portfolio.
Why both numbers matter together
- Tracking difference shows the cost. It’s the closest thing to an all-in measure of what following the index actually cost over that period, beyond just the stated expense ratio.
- Tracking error shows the consistency. A fund could look cheap on tracking difference in one year purely by luck, while having a track record of erratic tracking error that suggests the low number won’t repeat.
- Neither number alone tells the full story. Comparing funds using only one metric can be misleading, particularly for funds tracking less liquid or more complex indexes where the gap can vary widely period to period.
- Both are backward-looking. Past tracking behavior is a description of history, not a promise about how a fund will behave going forward.
The takeaway
Tracking difference tells you the size of the gap between a fund and its benchmark; tracking error tells you how steady that gap has been. Looking at both together, rather than treating a fund’s approximate similarity to its index as a single fixed fact, gives a fuller picture of how that similarity has actually behaved over time.