Tracking Error

What tracking error measures

The tracking error quantifies how precisely a passively managed fund or ETF actually replicates its reference index. Technically it is defined as the annualised standard deviation of daily return differences between the fund and the index, measured over a rolling window of typically 12 or 36 months. A low value means the investor really receives the index performance; a high value indicates systematic deviations and undermines the ETF's central promise.

In practice the figure for highly liquid vehicles on the MSCI World or S&P 500 sits at 0.05% to 0.15% per year — negligible. For exotic indices (emerging-market sectors, frontier-market small caps), tracking errors of 1% to 3% are not unusual.

Tracking error versus tracking difference

The two metrics are often confused but measure different things:

• Tracking difference (TD): the arithmetic difference between annual ETF return and index return. Example: index +7.00%, ETF +6.75% → TD = -0.25%. It tells you how much performance an investor gives up (or gains) relative to the index, net.
• Tracking error (TE): the standard deviation of daily return differences. It tells you how stable the tracking is — how reliably the ETF hugs the index.

For most retail investors, tracking difference is the more relevant number because it describes the effective performance gap after costs. Tracking error is more of a quality metric for fund management, and matters most to institutional users (pension funds, insurers) bound to narrow performance corridors.

Where deviations come from

Tracking error stems from several sources: running management fees (TER), transaction costs at index rebalancing, sampling effects under partial physical replication, withholding taxes on dividends of foreign equities, cash drag from inflows not yet invested, and FX differences for non-EUR vehicles. Synthetically replicating ETFs often minimise tracking error better than physical ones, at the cost of introducing counterparty risk.

Concrete observation

The iShares Core MSCI World UCITS ETF Acc (ISIN IE00B4L5Y983) showed a tracking error of 0.07% over 12 months in 2024 — against a TER of 0.20%. Tracking difference came in at -0.12% (the ETF actually beat the net index modestly, because iShares passes part of its securities-lending income back to the fund).

By contrast the iShares MSCI Frontier and Select EM UCITS ETF (ISIN IE00BQQP9G87) registered a tracking error of around 1.9% in the same year — driven by illiquid single markets (Vietnam, Bangladesh, Kazakhstan) and consequently elaborate sampling.

What investors often ask

Where do I find the tracking error of my ETF? In the provider factsheets under "Tracking", and in the semi-annual or annual report. The main comparison platforms are justETF and the ETFbook from the Vienna Stock Exchange.