What Causes an ETF to Have Tracking Error?
An index on paper is just a list of holdings and weights, but turning that list into an actual portfolio that trades in the real world introduces friction the index itself never has to deal with.
The short answer
ETF tracking error generally comes from a handful of sources: the fund’s expense ratio, the cost of buying and selling securities to stay aligned with the index, holding cash for dividends or redemptions, and sampling rather than owning every single index constituent. Each of these pulls fund performance slightly away from the benchmark, and they can work in different directions at different times.
Fees are the most predictable piece
The expense ratio is charged against the fund’s assets regardless of how the market performs, which creates a steady, mechanical drag relative to an index that has no costs at all. This piece of tracking error tends to be the most consistent and predictable of the bunch, since it’s a fixed percentage rather than something that varies with market conditions.
Trading costs add up quietly
Whenever an index changes its composition — a company is added, removed, or its weight is adjusted — the fund has to buy and sell to match, and those trades come with bid-ask spreads and sometimes market-moving costs, especially for less liquid securities. Frequent index turnover and rebalancing can meaningfully add to tracking error, particularly for funds tracking indexes with heavy churn.
Cash and sampling both introduce their own gaps
- Cash drag. Funds typically hold a small cash buffer to handle incoming dividends before reinvesting them or to meet investor redemptions, and that uninvested cash doesn’t participate in market moves the way the index’s full holdings do.
- Sampling risk. Some funds, especially those tracking very large or illiquid indexes, don’t hold every single constituent and instead hold a representative subset. This sampling approach can cause the fund’s behavior to diverge from the full index in ways that are hard to predict in advance.
- Foreign market timing. For funds holding international securities, a mismatch between when foreign markets close and when the US fund is priced can create apparent tracking gaps that aren’t really about the fund’s management at all.
- Offsetting income. Some funds partly counteract these drags through securities lending income, which can narrow — though rarely eliminate — the overall gap.
Why these effects don’t always point the same way
It’s tempting to assume all these factors push a fund below its index, but that outcome is not certain. Lending income, favorable sampling decisions, or lower-than-expected trading costs in a given period can occasionally offset fees enough that a fund’s return lands close to, or even slightly above, its stated benchmark before accounting for the index’s own theoretical assumptions. This is why tracking error is measured empirically rather than assumed from the fee alone.
A practical habit
Rather than assuming a fund’s expense ratio alone predicts how closely it will track its index, it can help to look at the fund’s actual historical tracking record, since fees are only one ingredient among several that determine the final gap. Past behavior is not a guarantee of future behavior, but it does show how these different forces have combined for that specific fund over time.