Tracking Error

Tracking error is the difference between an ETF’s actual returns and the returns of the benchmark index it is designed to replicate, typically measured as the annualized standard deviation of those daily return differences.

How Tracking Error Works

Every passive ETF aims to match the performance of an index — such as the S&P 500 — as closely as possible. In practice, no ETF matches its benchmark perfectly. The gap between what the ETF actually returned and what the index returned is called tracking error.

Tracking error is expressed as an annualized percentage. For example, a tracking error of 0.05% means the ETF’s daily returns deviated from the index’s daily returns by an average of 0.05% per year when measured statistically. A tracking error of 0% would mean perfect replication — which doesn’t exist in practice.

The most common cause of tracking error is the expense ratio: even a perfectly constructed ETF will lag its index by roughly its annual fee. Other causes include dividend reinvestment timing, index rebalancing costs, securities lending income, and cash drag (holding small amounts of cash that aren’t invested).

How Is Tracking Error Measured?

Tracking error is calculated as the annualized standard deviation of the difference in daily returns between the ETF and its benchmark. A lower tracking error indicates that the ETF is replicating its index more faithfully.

A closely related concept is tracking difference — the simple total-return gap between the ETF and the index over a given period (usually one year). Tracking difference answers “how much did the ETF underperform the index in absolute terms?” while tracking error answers “how consistently did it stay close?” Both are useful. For most long-term investors, tracking difference is the more intuitive figure.

For passive index ETFs, tracking error below 0.10% is generally excellent. Many large ETFs achieve tracking error of 0.02%–0.05%.

VOO (Vanguard S&P 500 ETF) consistently demonstrates tracking error below 0.03% — near-perfect replication of the S&P 500, partly because Vanguard’s securities lending income offsets some of the expense ratio cost.

IVV (iShares Core S&P 500 ETF) also maintains tracking error under 0.05% and has, in some years, slightly outperformed its benchmark index due to efficient portfolio management and lending income.

SPY (SPDR S&P 500 ETF Trust) has slightly higher tracking error than VOO or IVV, in part because its Unit Investment Trust structure prevents it from immediately reinvesting dividends.

Tracking Error vs Tracking Difference

Tracking error and tracking difference are related but distinct. Tracking difference is a single annual number: how much did the ETF return compared to the index? If the index returned 10.00% and the ETF returned 9.95%, the tracking difference is −0.05%.

Tracking error is a statistical measure of consistency: did the ETF stay close to the index every single day, or did it drift significantly before reverting? A fund can have a low tracking difference but high tracking error if it oscillated widely throughout the year before finishing close to the benchmark.

Related: [Tracking Difference]

Frequently Asked Questions About Tracking Error

What causes tracking error in ETFs?

The main causes are the expense ratio (which automatically creates a gap), dividend reinvestment timing (ETFs receive dividends from holdings but may hold cash briefly before reinvesting), index rebalancing costs, and fund structure. Some ETFs offset tracking error through securities lending — earning income by temporarily lending portfolio holdings to short-sellers — which can partially or fully compensate for the expense ratio drag.

Is tracking error the same as underperformance?

Not exactly. Tracking error measures statistical deviation from the benchmark, not direction. An ETF could technically show tracking error by occasionally outperforming the index as well as underperforming it. In practice, however, most passive ETFs lag their benchmark slightly due to fees and operational costs. For investors, the tracking difference (total annual gap) is usually more informative than tracking error alone.

How do I find an ETF’s tracking error?

ETF.com provides historical tracking difference and tracking error data for most ETFs, typically shown across multiple time periods. You can also calculate it yourself by comparing the ETF’s total return against its stated benchmark index return over the same period. Fund fact sheets from the issuer sometimes include this data as well.

RELATED_TERMS: Expense ratio, Net asset value, Index fund, Total return, Assets under management

Disclaimer: The information in this article is for educational and informational purposes only. It does not constitute financial, investment, or tax advice. Past performance is not indicative of future results. Investing in ETFs involves risk, including possible loss of principal. Always consult a qualified financial advisor before making investment decisions.