What Is Tracking Error?

An ETF or index fund is a product that promises to 'follow a certain index exactly.' For example, an ETF tracking the Nasdaq 100 is designed to rise when the Nasdaq 100 rises and fall when it falls.

But in reality it can't move 100% identically to the index. It drifts off slightly, in ways like the index rising +10% while the ETF rises only +9.6%. Tracking Error is the number that measures how much this 'drift' fluctuates day to day.

More precisely, tracking error calculates, as a standard deviation, how much the 'difference between the fund's return and the index's return' scatters each day, and it's usually annualized for display. The smaller the value, the more docilely it follows the index.

When you study ETFs, 'tracking error' and 'premium/discount' come up together and are easy to confuse. They are different concepts.

Tracking error looks at 'how well the ETF's actual net asset value (NAV) follows the underlying index.' It's a question of how well it was managed.

The premium/discount looks at 'how far the ETF's price traded in the market deviates from the ETF's actual net asset value (NAV).' That is, whether it's being traded more expensively, or more cheaply, than its underlying value in the market.

By analogy, tracking error measures 'how accurately the student copied down the textbook (index) content,' and the premium/discount measures 'whether that notebook sells at a fair price on the used market.' Both being small is seen as a sign of a good ETF.

The biggest reason tracking error and difference arise is, ultimately, 'costs.' An index is a number on paper, so it's free, but an ETF actually rolls along buying and selling stocks, so it costs money.

First, the management fee (total expense ratio, TER). This is the fee the manager takes. With a 0.20% annual fee, all else equal, it will lag the index by roughly that much. So for passive ETFs, this expense ratio is regarded as the best hint for gauging future tracking difference.

Second, rebalancing and trading costs. When the index changes its constituents, the ETF has to buy and sell along with it, and fees and taxes attach here.

Third, sampling. If the index holds too many stocks (especially thousands of bonds), the fund holds only representative names instead of all of them, which can widen the gap from the index a bit more.

Fourth, the cash balance (cash drag). Dividends are briefly held as cash before being rolled over again, and that cash can't rise as much as the index.

Fifth, exchange rates. An ETF tracking an overseas index has exchange-rate movements creeping in, which can widen the error.

Tracking error is usually shown in percent or in bp (basis points, 0.01%). A rough scale looks like this.

- Below about 0.05% (5bp): quite excellent for an index-tracking ETF - About 5–15bp: a common, typical level seen in most passive funds - Above about 20bp: possibly a sign of management issues or high costs

That said, these thresholds aren't absolute. Assets that are easy to buy and sell, like large U.S. stock indexes, have small errors, while assets that are hard to trade or involve exchange rates—like emerging markets, bonds, and commodities—naturally show larger errors. So it's meaningful to compare ETFs that track the same index.

Over a day or two, a 0.1% difference isn't even noticeable. But in long-term investing, this small gap compounds and changes the result.

For example, if you hold an ETF that steadily lags the index by 0.3% a year for 20 or 30 years, a noticeable difference appears in the final amount. For 'good assets, held long and steadily' to work, the very tool you use to follow the index must grip it well without leaks.

So when choosing among ETFs that track the same index, it helps to build the habit of looking not just at the name and reputation, but also at whether the expense ratio is low and whether the tracking error and difference are small and consistent. This isn't saying to buy a particular product—it's about cultivating the eye to check 'how faithfully it replicates the index.'