Tracking Error in Index Funds: Why Your Returns Differ
Your Nifty 50 index fund returned 11.62% last year while the index itself returned 12.40%. That 0.78% gap is not a mistake in your statement, and it is not a fund manager picking bad stocks. Tracking error measures how consistently an index fund’s returns deviate from its benchmark, expressed as the annualised standard deviation of the daily difference between the two.
The size of the gap is a separate number called tracking difference. Confusing the two is the most common mistake investors make when comparing passive funds.
What follows: how the two metrics differ, the five reasons a passive fund falls behind, a calculation you can reproduce on a calculator, and how to use both numbers when choosing between funds tracking the same index.
Tracking error and tracking difference are not the same
Tracking difference is the plain gap in returns: fund return minus index return. Above, that is minus 0.78%. It tells you how much you gave up.
Tracking error is about consistency, not size. It is the standard deviation of the daily return differences, annualised. A fund that lags by exactly 0.02% every single day has a large tracking difference over a year but almost no tracking error, because the shortfall is perfectly predictable.
Both matter. Tracking difference tells you the cost of ownership. Tracking error tells you how reliably the fund does its one job.
SEBI requires equity index funds and ETFs to disclose both, so you do not have to compute them yourself.
Why does my index fund not match the index exactly?
No index fund matches its benchmark perfectly, because the index is a mathematical construct with no costs and the fund is a real portfolio that pays for everything.
The expense ratio
The largest and most predictable drag, deducted daily from NAV in tiny slices. Hold a regular plan instead of the direct plan of the same scheme and you also pay a distributor commission, and that gap compounds. See how expense ratio affects NAV.
Cash drag
The fund receives SIP money and lump sums every working day, and that cash sits uninvested briefly. In a rising market uninvested cash earns nothing while the index climbs. In a falling market it helps. Cutting both ways is precisely why tracking error exists.
Rebalancing and corporate actions
When an index committee adds or drops a stock, the index switches at the closing price instantly and for free. The fund places real orders and pays brokerage, 0.1% STT on both legs of a delivery trade, stamp duty and market impact. Every rebalance leaves a dent.
Dividend timing
A total return index assumes dividends are reinvested on the ex-date. The fund receives the money days later, costing a few basis points in a rising market. If your fund is compared against a price index instead, the comparison itself is unfair, so check which benchmark the scheme uses.
Redemptions and portfolio limits
Large redemptions force selling regardless of price. Some funds cannot hold a stock at full index weight because of internal limits, and a few use sampling rather than full replication for very broad indices.
Worked example: calculating tracking error
Suppose you record the daily return difference between a fund and its index over five sessions, in basis points, where 1 basis point is 0.01%.
| Day | Fund return | Index return | Difference (bps) |
|---|---|---|---|
| Monday | 0.51% | 0.52% | -1 |
| Tuesday | -0.28% | -0.30% | +2 |
| Wednesday | 0.94% | 0.97% | -3 |
| Thursday | 0.15% | 0.15% | 0 |
| Friday | -0.62% | -0.60% | -2 |
Step one, the mean. Minus 1 plus 2 minus 3 plus 0 minus 2 equals minus 4. Divide by 5: the mean is minus 0.8 bps per day.
Step two, deviations from that mean: minus 0.2, plus 2.8, minus 2.2, plus 0.8, minus 1.2.
Step three, square and add: 0.04 plus 7.84 plus 4.84 plus 0.64 plus 1.44 equals 14.80.
Step four, divide by (5 minus 1) for the sample variance: 14.80 divided by 4 is 3.70, and its square root is 1.92 bps daily.
Step five, annualise by multiplying by the square root of 252 trading days, about 15.87. So 1.92 multiplied by 15.87 is roughly 30.5 bps, or 0.31% annualised tracking error. The method is the point here, not the figure: a real low-cost fund’s daily shortfall is far smaller than this sample.
What is an acceptable tracking error?
There is no single official threshold, and the reasonable range depends on the index.
A Nifty 50 fund holds 50 liquid stocks, so replication is cheap and the figure should be low. A midcap or smallcap index has less liquid constituents and wider spreads, so a higher number is normal. International and sectoral funds add currency and time zone gaps, and post the highest figures.
Rather than memorising a number, compare like with like:
- Compare only funds tracking the same index over the same period.
- Use at least one year of data. Three months will be dominated by a single rebalance.
- Check whether the disclosed figure is for the direct or the regular plan.
- Watch the trend. Rising tracking error in a fast-growing fund is a warning.
- Read tracking difference alongside it. Low error with a wide gap just means consistently expensive.
Also confirm what the scheme is trying to do at all. An active fund is meant to deviate, which active versus passive investing explains.
How to use these numbers when picking a fund
- Pick the index first. This decision matters more than the fund choice.
- List every scheme tracking it and note each expense ratio, direct plan only.
- Pull the disclosed tracking error and tracking difference for one year and three years from the factsheets.
- Drop any fund whose tracking difference is much wider than its expense ratio, because the extra gap is execution slippage you are paying for.
- Among the rest, prefer a larger asset base and higher traded volume, since scale reduces cash drag.
- Recheck once a year. Passive investing does not need monthly monitoring.
A short risk note: low tracking error means the fund follows its index faithfully. It says nothing about whether that index will make money. A near-perfect Nifty 50 fund still falls roughly as much as the Nifty in a correction.
Frequently Asked Questions
Is a lower tracking error always better?
For a pure index fund, yes, because the fund’s only mandate is to replicate. But read it with the tracking difference. A fund can post a very low tracking error while lagging the index by 1% a year, which means it is consistently costly rather than accurate. Both numbers together give the full picture.
Do ETFs have lower tracking error than index funds?
Often slightly, because an ETF does not handle daily investor cash directly, so cash drag is smaller. The catch shifts elsewhere: you buy an ETF on the exchange, so your own purchase price can sit above or below the underlying value. That difference does not appear in the fund’s tracking error at all, but it does affect your actual return.
Where can I find the tracking error of my fund?
The monthly factsheet and the scheme page on the AMC website carry it, since disclosure is mandatory for equity index funds and ETFs. AMFI hosts scheme level data as well. Confirm the period the figure covers and whether it refers to the direct or regular plan before comparing across fund houses.
Does a large fund size reduce tracking error?
Usually it helps, up to a point. A bigger fund spreads fixed costs thinner and receives steadier flows, which reduces cash drag. Very large funds tracking narrow or illiquid indices can face the opposite problem, where their own rebalancing trades move prices. Size is helpful for broad liquid indices, less so for niche ones.
Can an index fund beat its index?
Occasionally over short periods, yes, through favourable dividend timing, securities lending income or a rebalance executed at a better price than the index close. It is not repeatable and not a reason to choose a fund. Over multi-year periods, costs mean a passive fund lands slightly below its total return benchmark.
Key Takeaways
- Tracking difference is the size of the gap; tracking error is the annualised standard deviation of daily gaps. Read both.
- Expense ratio, cash drag, rebalancing costs including 0.1% STT, and dividend reinvestment lag explain almost all of the shortfall.
- SEBI mandates disclosure of both figures for equity index funds and ETFs, so check the factsheet rather than estimating.
- Only compare funds tracking the same index over the same period, using direct plan data, over at least one year.
- If the tracking difference is far wider than the expense ratio, the excess is execution slippage and a reason to look elsewhere.
- A low tracking error means faithful replication, not lower market risk: the fund will still fall with its index.
