You’ve decided to buy a Nifty 50 index fund. Smart. But there are 15+ Nifty 50 index funds from different AMCs. UTI, HDFC, ICICI, Nippon, SBI, Motilal Oswal… they all copy the same 50 stocks in the same proportion. So they should all give the same return, right?

They don’t. And the difference matters.

What is tracking error?

Tracking error is the gap between the index’s return and your index fund’s return.

If the Nifty 50 returned 14% this year and your index fund returned 13.7%, the tracking difference is 0.3%. Do that every year for 15 years and you’ve silently lost lakhs.

Tracking error is technically the standard deviation of these differences over time (how much the gap bounces around). Tracking difference is the cumulative gap. Most investors use “tracking error” loosely to mean both. What matters: your fund should trail the index by as little as possible, as consistently as possible.

Why can’t an index fund perfectly match the index?

The index is a mathematical construct. It doesn’t have real-world problems. Your fund does:

1. Expense ratio

The biggest and most obvious drag. If the fund charges 0.2% per year, that’s automatically 0.2% less than the index. This is guaranteed underperformance, built into the product.

But here’s the thing: expense ratio alone doesn’t tell you the whole story. Two funds can have the same expense ratio and different tracking errors. The operational stuff below explains why.

2. Cash drag

Cash drag is the return you lose because part of the fund’s money is sitting in cash instead of stocks.

The index assumes every rupee is invested at all times. A real fund can never do that. Two things get in the way.

Your SIP money waits before it becomes stocks.

You do a SIP on the 5th. The money reaches the AMC, gets a NAV allotted, and only then does the fund buy shares. That takes a day or two. If the market rose 1% in those two days, your money buys shares at the higher price, not the price on your SIP date.

Multiply this across lakhs of investors and every SIP date in the month. The fund is always holding fresh money that has not turned into shares yet.

The fund keeps cash ready to pay redemptions.

This is the part most people miss. When you redeem, the fund must pay you in a day or two. It cannot ask you to wait while it sells some Infosys.

The fund handles this by keeping about 1-2% of its money in cash. That cash pays the people who redeem that day. Without it, the fund would sell shares every single day. Every sale costs brokerage and tax. Keeping cash is cheaper.

But it still costs you. That cash goes into money market instruments. It earns about 4.5-5.25% overnight interest. The rest of the fund is in stocks earning 12-15%.

3. Dividend reinvestment timing

When a Nifty 50 company pays a dividend, the index assumes it’s reinvested instantly at the same price. In reality, the fund receives the dividend a few days later and reinvests it at whatever price prevails then. If the stock went up in those days, the fund missed the move.

4. Rebalancing lag

NSE reviews the Nifty 50 twice a year. Stocks that no longer meet the size and liquidity rules go out. Bigger, more traded ones come in. On the switch date, the index swaps them instantly. The fund has to place real orders in the market to sell the outgoing stock and buy the incoming stock. That takes time, and large orders cost money.

5. Corporate actions

Corporate actions are company events that change the number of shares or the share price:

• Stock split: one share becomes two or more. The price falls by the same ratio, so your total value stays the same. A ₹2,000 share split into two becomes two shares of ₹1,000.
• Bonus shares: the company gives extra free shares to people who already hold them. Get 1 bonus share for every 1 you own, and 10 shares become 20.
• Merger: two companies join into one. Your shares in the old company get swapped for shares in the new one.

The index adjusts for these instantly with a formula. The fund has to wait for the new shares to actually reach its account, which takes a few days. In that gap, the fund does not match the index.

What’s an acceptable tracking difference?

Use tracking difference for this check, not tracking error. Tracking difference is the plain return gap, so it already includes the expense ratio. That is the number that hits your money.

For a Nifty 50 index fund:

Tracking difference (annual)	Verdict
Under 0.2%	Excellent
0.2-0.4%	Good
0.4-0.7%	Average
Above 0.7%	Poor, consider switching

For context, if your fund has a 0.1% expense ratio, it should ideally trail the index by no more than 0.15-0.25% (expense ratio + minimal operational drag). If it’s trailing by 0.5%+ despite a low expense ratio, the fund house is operationally sloppy.

How to check the gap

Easiest: research sites. Value Research and Tickertape list tracking error under the “Risk” or “Peer Comparison” tab. Groww has an ETF screener that lets you sort funds by tracking error.

Most reliable: AMFI. SEBI requires every AMC to send daily tracking error data to AMFI, based on one-year rolling returns. AMFI publishes it as downloadable sheets covering all index funds and ETFs. Look under the “Other Data” section.

Most direct: the fund factsheet. Every AMC publishes a PDF factsheet by the 10th of each month. Find your fund’s page and look for the “Quantitative Indicators” or “Risk Metrics” block.

The expense ratio trap

Here’s a counterintuitive scenario:

Fund	Expense ratio	Tracking difference
Fund A	0.10%	0.40%
Fund B	0.18%	0.22%

Fund A looks cheaper. But Fund B actually gives you better returns because it tracks the index more tightly. The 0.08% you “save” on expenses is more than lost through operational inefficiency.

This happens because some smaller AMCs cut expense ratios to attract investors but don’t have the operational infrastructure (trading systems, cash management, corporate action handling) to track the index tightly.

Which Nifty 50 index funds track best?

As a general pattern (subject to change, verify before investing):

• UTI Nifty 50 Index Fund: Consistently among the tightest trackers. Large AUM helps with cash management.
• HDFC Nifty 50 Index Fund: Tight tracking, large fund house infrastructure.
• ICICI Pru Nifty 50 Index Fund: Large AUM, generally good tracking.
• Nippon India Nifty 50 Index Fund: Previously had higher tracking error, improved in recent years.

The differences are small (0.05-0.15% between the best and average). But over 20 years of compounding, 0.1% annually on a ₹50 lakh corpus is roughly ₹1-2 lakh. Not life-changing, but free money you’re leaving on the table.

ETFs vs index funds: tracking edition

ETFs (Exchange Traded Funds) often show lower tracking error than index mutual funds because they don’t face the same cash drag (ETFs use an in-kind creation/redemption mechanism). But ETFs have their own costs:

• Brokerage when you buy/sell
• Bid-ask spread (you might pay slightly more than NAV when buying)
• Demat account required
• Liquidity issues (some ETFs barely trade, so the spread can be 0.5-1%)

For most retail investors, an index mutual fund with tight tracking is simpler and more practical than an ETF. We cover this comparison in detail in our ETF vs Index Fund guide.

Key takeaways