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Tracking Difference in Practice: How to Verify an ETF Truly Follows Its Index
Key takeaways
- Tracking difference (TD) measures the actual difference between a fund's return and the index over a full year — it is more precise than TER alone.
- A negative TD means the fund outperformed the index; a positive TD means it lagged.
- TD can be found in fund documents (KIID, annual report) or at trackingdifferences.com.
- Compare TD across multiple years — one year may be an anomaly.
- A low TER does not guarantee a low TD; dividend tax optimisation can improve TD.
Tracking difference (TD) is the difference between an ETF's annual return and that of its benchmark index — and it is the most rigorous measure of how faithfully a fund actually replicates its index.
Why looking only at TER is not enough
TER (Total Expense Ratio) is the annual fee charged by the fund. It is the number you see in every comparison tool. But TER does not account for how the fund handles dividends, how it replicates the index, or the transaction costs incurred when the index is rebalanced. The result may be a fund with a TER of 0.20% and a TD of 0.35% — and a competitor with a TER of 0.15% and a TD of 0.05%. For an investor, the second fund is the better deal.
What affects TD
- Dividend optimisation: Irish-domiciled funds pay a lower withholding tax on US dividends (15% instead of 30%), which improves TD. More on this in the article why Irish domicile.
- Securities lending: Funds can lend out shares and return the income to investors — TD can even be negative.
- Replication method: Physical (full or optimised sample) vs. synthetic replication are reflected differently in TD.
- Index rebalancing: Frequent changes to index composition mean more trades and costs.
How to use TD practically when selecting a fund
When comparing two funds tracking the same index, look at TD over the last 3–5 years. You are looking for consistency — a fund that has a low TD every year is more reliable than one that excels one year and lags the next. A TD above 0.30% for a large, liquid index such as the S&P 500 or MSCI World is a warning sign.
Quick checklist
- What is the fund's average TD over the last 3 years?
- Is the TD consistent, or does it fluctuate?
- How does the fund compare to direct competitors tracking the same index?
- Where is the fund domiciled and how does that affect dividend taxation?
Tracking difference is a less visible figure than TER, but for a long-term investor it is a more accurate indicator of true costs. Find more about ETF selection in the ETF guide.
FAQ
What is tracking difference in simple terms?
The annual difference between what the fund earned and what its benchmark earned. A negative TD means the fund outperformed the index. It tells you how much the replication actually costs you — or earns you.
How does TD differ from tracking error?
Tracking error measures the volatility of deviations between the fund's return and the index, not the deviation itself. TD shows the total annual difference. For a long-term investor, TD is the more practical figure.
Can an ETF have a negative tracking difference?
Yes, and it is not uncommon. Irish funds with dividend-tax optimisation or funds that actively lend out securities can outperform the index by a small margin. A negative TD is favourable for the investor.