What You'll Learn

I’ve been investing in ETFs for over a decade, and one metric I wish I’d paid attention to earlier is tracking error. When I first started, I assumed all ETFs tracking the same index would perform identically. Boy, was I wrong. In 2020, I owned two emerging market ETFs that both claimed to follow the MSCI Emerging Markets Index. One returned 12% that year; the other returned 9.5%. The difference? Tracking error. In this guide, I’ll break down exactly what tracking error is, why it eats into your returns, and how to avoid the worst offenders.

Key Insight: Tracking error isn't just a number on a fact sheet. Over 10 years, a 0.5% tracking error can compound into thousands of dollars lost — far more than the difference in expense ratios.

What Is Tracking Error?

Tracking error measures how consistently an ETF follows its benchmark index. Technically, it’s the standard deviation of the difference between the ETF’s returns and the index’s returns over a given period. A low tracking error (say, 0.1% annualized) means the ETF closely mirrors the index. A high tracking error (above 1%) means the fund often deviates — sometimes for better, sometimes for worse.

Think of it like a race car: the index is the ideal racing line. Tracking error tells you how often your car drifts off that line. Even small drifts add up over a long race.

Why Tracking Error Matters More Than Fees

Most investors obsess over expense ratios. But a fund with a 0.03% expense ratio can still have a 0.5% tracking error due to inefficient replication, cash drag, or securities lending practices. In many cases, the tracking error cost exceeds the fee. I’ve seen people switch from a 0.10% fee fund to a 0.03% fee fund, only to lose more in tracking error.

Real Example: In 2021, I compared two S&P 500 ETFs: Fund A (ER 0.03%, tracking error 0.08%) and Fund B (ER 0.02%, tracking error 0.25%). Over 3 years, Fund A outperformed Fund B by 0.14% annually, despite having a higher fee. The tracking error gap more than erased the fee advantage.

How to Calculate Tracking Error (With a Real Example)

You can calculate tracking error yourself using daily return data. Here's the formula in plain English:

Tracking Error = Standard Deviation (ETF Return – Index Return) × √(Trading Days per Year)

I pulled daily data for the iShares Core S&P 500 ETF (IVV) and the S&P 500 index for 2023. The daily differences averaged 0.01%, and their standard deviation was 0.03%. Multiply by √252 (approx 15.87), and we get an annualized tracking error of about 0.48%. IVV's official tracking error is around 0.04%, so my manual calculation was off — but that's because I didn't adjust for fund distributions. The point is, you can do this yourself.

Common Causes of High Tracking Error

Through my own pain and research, I’ve found these recurring culprits:

  • Sampling vs. full replication: Some ETFs don't hold every stock in the index. Especially in niche markets (small-cap, emerging), they sample. Sampling introduces tracking error because the chosen stocks may behave differently.
  • Cash drag: Funds hold cash to handle redemptions. In a rising market, cash drags returns down. In a falling market, cash can help. Either way, it creates divergence.
  • Securities lending: Lending shares generates extra income (reducing tracking error) but also introduces counterparty risk and can cause small timing mismatches.
  • Replication method: Synthetic ETFs (using swaps) can have different tracking characteristics than physical ones.
  • Currency hedging costs: For international ETFs, hedging currency exposure can be expensive and imperfect, adding to tracking error.

How to Choose an ETF With Low Tracking Error

I use a three-step process before buying any ETF:

  1. Check multiple timeframes: Look at 1-year, 3-year, and 5-year tracking error figures. A fund with consistently low tracking error across periods is more reliable.
  2. Compare with peers: I built a simple table when I was choosing between emerging market ETFs:
ETFExpense Ratio1-Yr Tracking Error3-Yr Tracking Error
Fund X0.15%0.22%0.19%
Fund Y0.09%0.45%0.52%
Fund Z0.20%0.08%0.11%

Fund Z had the highest fee but the lowest tracking error. Over 5 years, its total cost (fee + tracking error) was actually lower than Fund Y's. I went with Fund Z.

  1. Read the prospectus: Some ETFs disclose their tracking error target. Look for phrases like “designed to track the index within 0.05%”.
Pro Tip: Don't rely only on annualized figures. Check the maximum deviation in any given month. A fund with tight average tracking error but huge monthly spikes can wreck your rebalancing strategy.

Frequently Asked Questions

I own an S&P 500 ETF with 0.03% fee but its tracking error is 0.25%. Should I switch?
First confirm the tracking error figure is annualized and includes dividend timing. If it’s genuine, switch to a lower tracking error fund like IVV or VOO. The fee difference is negligible, but the tracking error gap will compound. I once stuck with a high tracking error fund for “low fees” and regretted it.
Why does my bond ETF have higher tracking error than my stock ETF?
Bond indices are harder to replicate because bonds trade over the counter and prices are less transparent. Also, bond ETFs often use sampling. Expect tracking errors of 0.3%–0.8% for bond ETFs vs. 0.05%–0.2% for large-cap stock ETFs. Nothing to panic about unless it exceeds 1%.
Can a high tracking error ever be beneficial?
Rarely, and only in very specific cases like when an ETF uses optimization that accidentally overweights winners. But over the long run, consistent positive tracking error is a mirage. The fund’s prospectus states it aims to track the index, not beat it. If you want alpha, buy an active fund.
How often should I check tracking error for my ETFs?
I check annually or after any major market event. Daily monitoring is overkill. But if your ETF changed its replication method or management team, check the tracking error for the next 6 months to see if it drifted.
Does tracking error matter for leveraged or inverse ETFs?
Absolutely, but it works differently. Leveraged ETFs target a multiple of daily returns, so their tracking error over longer periods is huge due to compounding. Don't hold them for more than a day. For them, tracking error is almost always detrimental.

This article is based on personal experience and data verified through Morningstar and fund prospectuses. Always verify current tracking error data before making decisions.