Expense Ratio vs. Tracking Error: Why Aren't They the Same?
It’s a natural assumption that the fund with the lowest fee will track its index the most closely, but fees are only one part of a longer list of forces at work.
The short answer
The expense ratio is a fixed, disclosed annual fee a fund charges, while tracking error measures how much a fund’s actual return has varied from its benchmark over time. A low expense ratio tends to help keep tracking error smaller, but it doesn’t determine it entirely, because other factors like trading costs, sampling decisions, and securities lending income also shape the final gap.
Why fees alone don’t explain the whole picture
The expense ratio is predictable and disclosed upfront, which makes it an easy number to compare across funds. But it only captures the fund’s stated management and administrative costs. It doesn’t capture the cost of buying and selling securities to keep pace with index changes, the effect of holding uninvested cash, or income the fund may earn from lending out its holdings. All of these can move a fund’s real-world tracking error above or below what the expense ratio alone would suggest.
How the two numbers can diverge in practice
- A low-fee fund with high tracking error. A fund could charge very little but track an index with frequent turnover or illiquid holdings, leading to trading costs that add more drag than the fee itself.
- A higher-fee fund with low tracking error. A fund with a somewhat higher expense ratio could still track its benchmark very consistently if it uses efficient trading practices or benefits from steady lending income.
- Sampling adds its own variable. Funds that use sampling rather than full replication introduce a source of tracking error that has nothing to do with the fee at all.
- Structural differences matter too. Bond funds, for instance, often show higher tracking error than stock funds for reasons tied to the bond market’s structure, not the fee charged.
Why both figures are worth checking
Expense ratio tells you the known, contracted cost of owning a fund. Tracking error tells you how that fund has actually behaved relative to its benchmark once all the other real-world frictions are included. Relying on the fee alone assumes those other frictions are negligible or identical across funds, which isn’t always a safe assumption, particularly for funds tracking narrower or less liquid corners of the market. Two funds tracking the very same index can post noticeably different tracking error even with nearly identical expense ratios, simply because they make different choices about trading, sampling, or lending.
The takeaway
A fund’s expense ratio is a useful, predictable starting point, but it’s not a complete substitute for looking at its actual historical tracking record. Comparing both figures side by side, rather than assuming the lowest fee automatically means the closest tracking, gives a more grounded basis for evaluating how a fund has performed relative to its benchmark over time and across different market conditions.