Two index ETFs can lag the same benchmark by the same amount on average while behaving very differently from month to month. ETF tracking difference tells you how far the fund finished from its benchmark over a stated period. Tracking error tells you how much the return gap varied across observations. To judge how closely a fund followed an index, look at both figures and check how each was calculated.
What ETF tracking difference measures
Subtract the benchmark’s total return from the fund’s total return over the same period. If the ETF returned 7.80% and the benchmark returned 8.00%, tracking difference was −0.20 percentage points, or −20 basis points. A positive result means the fund finished ahead using this sign convention. If a factsheet defines the gap as index minus fund, the sign reverses. Read the definition before comparing figures across providers. Vanguard explains the fund-minus-benchmark convention and its drivers.
A trailing one-year difference uses the two total returns for that year. For a multiyear comparison, check whether both numbers are cumulative or annualized. If you have monthly returns, compound each series separately by multiplying one plus each monthly return, then subtract the two results. Adding monthly gaps gives a different measure because each month starts from a different base.
What ETF tracking error measures
Tracking error is commonly the annualized standard deviation of periodic fund-minus-benchmark returns. Standard deviation measures how far those observations spread around their own mean. A fund can miss by a similar amount every month and still have low tracking error. A fund whose monthly gaps swing from positive to negative can show higher tracking error even if its cumulative return ends up close to the benchmark.
To reproduce a conventional historical calculation, align the fund and benchmark observations, subtract one from the other for each period, and calculate their standard deviation. If you use monthly data and sample standard deviation, one common annualization convention is to multiply by the square root of 12. State the frequency, observation window, and convention alongside the number. Some disclosures use different methodologies, and a short sample offers limited evidence about future behavior.
A worked example with two hypothetical ETFs
Suppose both funds follow the same index. The table shows monthly total returns in one currency. The gaps are in percentage points.
| Month | Benchmark | Fund A | A gap | Fund B | B gap |
|---|---|---|---|---|---|
| 1 | 1.00% | 0.90% | −0.10 | 1.20% | +0.20 |
| 2 | −2.00% | −2.10% | −0.10 | −2.40% | −0.40 |
| 3 | 2.00% | 1.90% | −0.10 | 2.20% | +0.20 |
| 4 | 0.50% | 0.40% | −0.10 | 0.10% | −0.40 |
Both funds average a monthly gap of −0.10 percentage points. Fund A’s gap is identical in every observation, so its sample standard deviation is zero. Fund B alternates between +0.20 and −0.40 points. Its monthly sample standard deviation is about 0.35 percentage points. Annualizing the unrounded result gives 1.20 percentage points. The four-month figures are an illustration of the calculation. A real comparison needs a longer period and the same measurement dates for both funds.
Compounding the returns gives approximately 1.46% for the benchmark, 1.06% for Fund A, and 1.05% for Fund B. The funds end the period about 0.40 and 0.42 percentage points behind the benchmark, respectively. Their endpoints are similar, while their monthly gaps tell very different stories.
Why the gap changes
Fees can create a persistent drag, while index rebalances, transaction costs, cash balances and sampling can affect the gap unevenly. Securities lending income can offset some expenses. Timing differences can also matter. For example, a fund and an index might value overseas holdings at different times, producing a gap that partly reverses in the next observation. Treat an isolated spike as a reason to investigate the dates and valuation rules before attributing it to portfolio management. Vanguard’s index tracking guide lists common sources of return differences.
How to compare fund disclosures
- Match the benchmark. Check the exact index variant, including currency, hedging, and whether the series uses price, gross total, or net total returns.
- Match the fund series. Check that both funds use NAV total returns and the same distribution treatment. Market-price returns can also reflect changes in the premium or discount to NAV.
- Match the dates. Use identical start and end dates. For tracking error, also match daily, weekly, or monthly frequency and confirm the reported annualization.
- Separate trading costs. If you are estimating your own trading cost, examine the spread and execution price separately. See the ETF bid-ask spread guide.
What to write down before drawing a conclusion
Keep a short record of the fund ticker, share class, index name, dates, return series and calculation method. That record makes it possible to distinguish a real change in tracking from a change in the source data. For instance, comparing a fund’s net return against a gross index one year and a net index the next can move the reported gap even when the fund’s management process stays the same. If two sources disagree, reproduce the calculation from their published inputs before relying on the difference.
When comparing two funds, start with their return gaps on the same basis. Then use tracking error to see whether those gaps were steady or uneven. If the disclosures use different periods or conventions and comparable inputs are unavailable, leave the figures side by side with their labels rather than treating them as directly comparable.
