Understanding Tracking Error in Index Funds
An index fund should mirror its index — but never perfectly. Tracking error measures the gap. Here’s why it matters.
On this page
A passive index fund aims to replicate its benchmark, but it can never match it perfectly. Tracking error measures how much the fund’s returns deviate from the index — a key quality metric for passive funds.
Why funds drift from the index
- Expense ratio — fees are deducted, so the fund lags the index by roughly that amount.
- Cash drag — a small cash balance for redemptions isn’t fully invested.
- Rebalancing & replication — delays and costs when the index changes constituents.
- Dividends — timing of reinvested dividends.
Tracking error vs tracking difference
- Tracking difference — the simple return gap between fund and index over a period.
- Tracking error — the volatility of that gap (how consistently the fund tracks). Lower is better on both.
How to use it
When choosing between two index funds on the same index, prefer the one with lower tracking error and lower cost — it follows the benchmark more faithfully. A fund with high tracking error isn’t doing its one job well.
For ETFs, also watch the market-price gap to NAV in thin trading. Tracking error is small for good index funds, but over decades even small, consistent drift compounds. Education only, not a recommendation.
