Two Funds Follow Your Index Differently
Two funds tracking the same index sit side by side on a retirement account shortlist. Their fees look close, and their latest annual returns offer little reason to prefer either. A closer look at their monthly results could reveal a less obvious contrast.
Imagine one fund staying slightly behind the index almost every month, while the other alternates between leading and lagging. The choice becomes harder: is a steady shortfall preferable to an uneven path that might leave less money behind overall?
Tracking error measures how variable those return gaps are; tracking difference measures the fund’s return minus the index’s over a period. Consistency alone does not establish superiority.
Does a Smaller Tracking Error Mean Better Returns?
The steadier fund may look more dependable without delivering the better return. A regular deduction for fees can leave returns consistently below the benchmark while adding little variability to the return gaps. Low tracking error can therefore coexist with a persistent cost to investors; it does not establish that a fund finishes closer to the index.
Consider a hypothetical year in which the index returns 8%. Fund A returns 7.6%, with relatively stable monthly gaps. Fund B returns 7.9%, but its monthly gaps fluctuate more. Fund A could have the lower tracking error, while Fund B has the smaller annual shortfall: 0.1 percentage point rather than 0.4. On a $10,000 starting investment, those returns leave Fund B ahead by $30. The example separates the consistency of tracking from the return actually received.
For a long-term comparison, lower tracking error is evidence of more consistent relative performance, not automatically better performance. The larger fluctuations in Fund B also offer no assurance of a smaller future shortfall. Before treating either figure as an advantage, the comparison needs matching benchmarks and measurement periods.
Check the Measurement Before Trusting the Number
A factsheet showing 0.10% tracking error beside another showing 0.20% may appear to settle the comparison. Before ranking them, inspect the calculation notes: the observation period, return frequency, and whether the figure is annualized. A daily calculation over one year is not directly comparable with a monthly calculation over three years. If those details are missing, the apparent advantage remains unverified.
Then check the exact benchmark version and reporting currency, not merely the index name. A price index excludes ordinary dividend income; total-return versions incorporate reinvestment, with gross and net versions differing in their treatment of dividend withholding taxes. Comparing a fund’s total return against the wrong version can make a measurement mismatch look like superior or inferior tracking. Both funds need the same benchmark basis and matching start and end dates.
For ETFs, also distinguish net asset value returns from market-price returns. Changes in premiums or discounts can introduce deviations unrelated to portfolio replication. Use a consistent return basis before judging either fund. Once those mismatches are removed, a recurring shortfall becomes more meaningful—and needs an explanation beyond the tracking-error figure.
A Persistent Shortfall Needs a Different Explanation
If the fund repeatedly trails its benchmark, compare that gap with its annual expense ratio. A hypothetical fund charging 0.15% and falling behind by roughly 0.15 percentage point a year presents a plausible fee-driven pattern. A recurring 0.40-point shortfall deserves closer inspection, but the extra gap does not, by itself, prove poor management.
The expense ratio is only one influence on returns. Trading costs and cash awaiting investment can deepen the shortfall; securities-lending income retained by the fund can offset part of it. Holding a representative sample rather than every index security can also create differences, though those differences need not always be negative. The observed gap therefore cannot be treated as a precise bill for fees alone.
Read the fund’s reports for an explanation that fits the pattern, rather than assuming the cheapest published fee wins. An unexplained residual shortfall leaves uncertainty about implementation. A reasonably explained, steady gap narrows that uncertainty—but says little about what happens when deviations start fluctuating.
What Changes When Deviations Become Uneven?
Once the gaps widen and narrow unpredictably, the concern shifts from the size of the shortfall to the reliability of benchmark exposure. Higher tracking error signals greater variability around the fund’s average relative return—not necessarily a larger loss or greater overall market risk. A fund can finish close to its index while taking a less consistent path.
Inspect when the larger deviations occur. A representative sample can behave differently from the full index as individual securities move; cash awaiting investment and the timing of index changes can also disturb tracking. These implementation constraints can produce uneven gaps without a change in the published fee. Their presence offers possible explanations, not proof of the cause.
For someone planning a withdrawal or portfolio rebalance, an unfavorable gap at that moment would matter even if it later reversed. The comparison now needs evidence that the observed pattern extends beyond a favorable reporting period.
One Good Year Is Not Enough Evidence
A narrow annual gap can conceal several months of offsetting deviations. Sampling differences may help relative returns in one period and hurt them in another, so a favorable year does not establish a repeatable advantage. That makes a longer record worth examining before treating the latest result as evidence of better tracking.
Compare several matching calendar years alongside rolling periods, keeping the benchmark and calculation basis unchanged. Look for recurring shortfalls and whether tracking error stays broadly stable across different market conditions. Check when fees or replication methods changed; older observations may describe an approach the fund no longer uses. A newer fund presents the opposite constraint: too little history to test persistence. Treat its encouraging figures as provisional. The useful distinction is between an observed pattern and a dependable expectation—not between one good year and one bad year.
Use Both Measures to Set Realistic Expectations
The shortlist can now reflect a trade-off rather than a ranking by one number. With benchmarks and measurement periods aligned, weigh the recurring tracking difference against its variability. A smaller shortfall may suit a long-term holding, while lower tracking error may matter more when consistent benchmark exposure is the priority. Neither removes the costs of implementation.
Use the observed pattern as a planning assumption, not a promised annual result. An unexplained shortfall or limited history should leave more room for uncertainty. The preferred fund need not match the index perfectly; its deviations should be understandable, acceptable, and worth monitoring as the holding continues.
