Tracking error measures how closely an index fund or ETF actually follows its index, specifically, how much the fund's returns wobble around the index's returns over time. A fund that promises to copy the Nifty 50 never copies it perfectly, and tracking error is the number that tells you how imperfect the copy is.
This article assumes you know what an index fund is, if not, start with what is an index fund, and how is it different from an ETF?
Why doesn't a fund match its index exactly?
The index is a calculation; the fund is a real portfolio with real-world frictions. Three of them do most of the damage.
Costs. The index pays no fees. The fund charges an expense ratio and pays brokerage, taxes and other charges when it trades. Every rupee of cost comes straight out of the fund's return.
Cash drag. The index is always 100% invested. It's just arithmetic. The fund must keep a small cash buffer for investor redemptions, and fresh money takes a little time to deploy. When the market rises, that idle cash earns nothing and the fund lags; when the market falls, the same cash softens the fall slightly. Either way, the fund has drifted from the index.
Rebalancing lag. When the index swaps stocks. The process described in what is index rebalancing, and why do stocks move on it?, the index switches instantly at the official price. The fund has to place actual orders, cross the bid-ask spread, and may get slightly different prices. Dividends create a similar gap: the index assumes instant reinvestment, the fund reinvests when the cash actually arrives.
What does it look like in numbers?
Arjun compares a Nifty 50 index fund with its index over three illustrative years:
| Year | Index return | Fund return | Difference |
|---|---|---|---|
| 1 | +12.0% | +11.6% | −0.4% |
| 2 | −5.0% | −5.3% | −0.3% |
| 3 | +8.0% | +7.5% | −0.5% |
Two related ideas hide in that table:
- Tracking difference is the simple gap each period (−0.4%, −0.3%, −0.5%). It's usually negative, because costs only subtract.
- Tracking error is, strictly, the variability of that gap, how much it jumps around. A fund that lags by almost exactly 0.4% every year has a low tracking error even though it always lags; a fund whose gap swings between +0.2% and −1.1% has a high tracking error, because you can't predict how far off the copy will be.
For everyday comparisons, both numbers matter: the average gap tells you the cost of the copy, the wobble tells you how sloppy it is. All figures above are illustrative, not any real fund's record.
How do I use this as an investor?
When two funds track the same index, they own essentially the same stocks, so the tidiness of the copy becomes a genuine point of comparison. Fund houses publish tracking error and tracking difference in scheme documents and factsheets. Persistent, unusually large gaps versus peers tracking the same index are worth understanding before you invest: they usually trace back to higher costs, larger cash buffers or clumsier execution.
Small gaps, though, are normal and unavoidable. Expecting a fund to match its index to the decimal is expecting a photocopy with zero toner cost.
Things to keep in mind
- Some gap between an index fund's return and its index is built into how funds work, zero tracking error doesn't exist in practice.
- Compare tracking numbers only between funds following the same index; across different indices the comparison means nothing.
- A low expense ratio helps but isn't the whole story, cash drag and execution quality also feed the gap.
- Check the scheme's published tracking error/difference over multiple periods, not one flattering year.
Read next
What are beta and alpha? — Finally, the two measures for judging any portfolio against its index.