Tracking difference: how closely an ETF follows its index
An index ETF sets out to copy an index. In practice it lands a hair above or below. That small gap has a name: tracking difference.
Copying an index is harder than it sounds
An The published list of investments (the “index”) the fund aims to copy, such as the MSCI World. More → is just a published list — say, the 1,500 companies in a broad world index. An ETF tries to hold those companies so its value moves in step. But a few things get in the way every year, so the fund’s actual return ends up a little above or below the index. Measured over a period, that gap is the tracking difference. It is distinct from the fund’s yearly fee, though the fee is one of the main causes. A fund might charge 0.2% in fees but land only 0.1% below its index in a given year — or, in a good year for securities lending, actually beat its index slightly. The tracking difference is the outcome; the fee is just one of the inputs that shape it.
The main causes of the gap
Several things nudge a fund away from its index each year. The The yearly running cost of the fund, shown as a % of your money. Lower is cheaper. More → is the most predictable drag — it is deducted continuously from the fund’s assets. Dividend withholding tax is less obvious: when companies in the fund pay dividends, the fund sometimes owes tax on those dividends before reinvesting them, whereas the index typically assumes dividends arrive gross and are reinvested in full. The How the fund copies its index: by buying the shares directly (physical) or using a swap contract (synthetic). More → also matters: a fund that holds every index member pays trading costs every time the index rebalances; one that holds a representative sample may trade less but introduces a sampling gap. Each of these effects is usually small; combined over a year, they determine how closely the fund tracks its benchmark.
Securities lending can push it the other way
Not every force nudges the fund below its index. Many ETFs earn additional income by temporarily lending out the shares they hold to short-sellers and other borrowers, in exchange for a fee. That income flows back into the fund and can offset some or all of the other drags. In some years, for some funds, the lending income more than covers the fee and tax drag, so the fund ends up above its index for the period. That sounds like a free lunch, but it comes with a small counterparty risk: if the borrower cannot return the shares, the fund faces a shortfall, mitigated by collateral but not reduced to zero. Large, well-run funds manage this risk carefully; it is worth knowing it exists rather than assuming an above-index return is guaranteed to repeat.
Where to find the data
The most reliable source is the fund issuer’s own website or its annual report, where it publishes the fund’s Performance that includes reinvested dividends — the fuller picture of what you actually earned. More → alongside the index return for the same period. The gap between the two is the tracking difference. Third-party tools such as justETF or the comparison features on major fund platforms often do this calculation for you and display it across multiple years. Looking at several years matters more than any single year: a one-year snapshot can be distorted by unusual events, whereas a three-to-five-year picture shows how the fund has consistently behaved across different market conditions and index-rebalancing cycles.
How to read it
For a plain index ETF, a sensible quality check is that the tracking difference is small and fairly steady from year to year — ideally close to the fund’s stated fee or better. A gap that is large relative to the fee, or that swings around unpredictably, is a prompt to investigate: is there a one-off explanation, or does the fund consistently struggle to copy its index? A large or erratic gap is worth digging into, but it is not a verdict on its own. And past tracking data, like past returns, does not promise the future — it is a quality check, not a guarantee of future faithfulness.