What Is Tracking Error?
Tracking error measures how closely an ETF follows its benchmark index. Specifically, it is the standard deviation of the daily return differences between the ETF and its index. A tracking error of 0.02% means the ETF's daily returns deviate from the index by an average of 0.02 percentage points.
A closely related concept is tracking difference, which measures the total cumulative return gap over a specific period. If an index returns 10.00% in a year and the ETF returns 9.92%, the tracking difference is -0.08%.
Both metrics matter. Tracking difference tells you how much return you missed. Tracking error tells you how consistently the ETF followed the index day to day. The best index ETFs minimize both.
Tracking Error vs Tracking Difference: What Is the Difference?
These two terms are often confused, but they measure different things:
Tracking difference is a single number: the total return gap between the ETF and its benchmark over a period. It answers the question, "How much did I lose (or gain) compared to the index?" It is typically negative and close to the negative of the expense ratio.
Tracking error is a volatility measure: the standard deviation of daily return differences. It answers, "How consistently did the ETF follow the index each day?" A low tracking error means the ETF's daily returns consistently match the index, even if they are all slightly lower due to fees.
An ETF can have a small tracking difference but high tracking error. This would mean that the total return was close to the index, but the day-to-day returns were erratic. Ideally, you want both to be low.
What Causes Tracking Error?
Several factors contribute to tracking error in index ETFs:
Expense ratio: The biggest and most predictable cause. The fund deducts fees daily, creating a constant slight drag on returns versus the fee-free index. An ETF with a 0.03% expense ratio will naturally trail its index by about 0.03% per year, all else equal.
Sampling: Some ETFs do not hold every security in the index. An ETF tracking a broad index with thousands of small, illiquid stocks may hold a representative sample instead. This introduces deviations from the index return.
Cash drag: ETFs may hold a small amount of cash for operational reasons (handling dividend payments, meeting redemptions). Cash earns minimal returns compared to the index's securities, creating a slight performance drag.
Rebalancing timing: When the index adds or removes stocks, the ETF must adjust its holdings. If the ETF cannot trade at the exact same prices used to calculate the index change, small differences accumulate.
Securities lending income: Many ETFs lend out shares of their holdings to short sellers, earning lending fees. This income can partially or fully offset the expense ratio, sometimes reducing the tracking difference to near zero — or even turning it positive.
What Is an Acceptable Tracking Error?
Acceptable levels depend on the type of ETF:
Large-cap US stock ETFs (e.g., S&P 500 funds like VOO or IVV): Tracking error should be under 0.05%. These funds hold liquid stocks and can closely replicate their index.
Broad US stock ETFs (e.g., total market funds like VTI): Under 0.05% is excellent. Slightly higher than large-cap-only funds due to small-cap holdings.
International developed ETFs (e.g., VEA, VXUS): 0.05%–0.15% is typical. Time zone differences and currency effects contribute to higher tracking error.
Emerging market ETFs (e.g., VWO): 0.10%–0.30% is common. Less liquid markets, capital controls, and sampling make precise tracking harder.
Bond ETFs: 0.05%–0.20% for broad investment-grade funds. Bond indexes contain thousands of securities, many illiquid, making full replication impractical.
How to Evaluate an ETF's Tracking
When comparing ETFs that track the same index, follow these steps:
First, compare tracking difference over multiple periods (1-year, 3-year, 5-year). A consistently smaller tracking difference means you keep more of the index's return. Do not rely on just one year — look for consistency.
Second, check if the tracking difference is close to the negative of the expense ratio. If the ETF trails the index by significantly more than its expense ratio, something else is dragging on performance (poor execution, excessive cash, high turnover costs).
Third, look for negative tracking difference that is less than the expense ratio. This usually means the fund is earning securities lending income that offsets some of the fee drag. Some S&P 500 ETFs actually outperform their after-fee expected return because of this.
Use the ETF comparison tool to see how specific funds stack up on these metrics.
Why Tracking Error Matters for Your Portfolio
For a single ETF, the impact of tracking error on your returns is usually small — a few basis points per year. But it matters for two reasons:
First, it compounds over time. A consistent 0.10% annual tracking difference means you lose roughly 1% of returns over a decade. This is on top of whatever the expense ratio costs you.
Second, it is a quality signal. High tracking error often indicates operational issues with the fund — poor execution, inadequate resources, or structural problems. A fund that cannot track its index efficiently may have other issues you want to avoid.
For core portfolio holdings that you will own for decades, it is worth spending a few minutes checking tracking error. The data is freely available and can help you distinguish between funds that look similar on the surface but differ in execution quality. Browse S&P 500 ETFs to compare some of the most popular index trackers.
