Tracking Error Explained
Tracking Error Explained
Tracking error is the difference between a fund's return and its benchmark index. Even low-cost index funds have tracking error; understanding its causes helps you pick the best funds.
Key takeaways
- Tracking error is typically 0.01-0.10% annually for quality index funds; anything higher suggests operational inefficiency
- Sampling (holding a subset of index holdings) creates small tracking error unavoidable at scale
- Cash drag, rebalancing frequency, and corporate action timing all contribute to tracking error
- Lower tracking error is generally better, but tiny differences (0.01-0.02%) are negligible
- The best index funds (VTI, FSKAX, SCHZ) have tracking error under 0.05%, indistinguishable from perfect index replication
What tracking error is
Tracking error is the annualized standard deviation of the difference between a fund's return and its benchmark index return.
In practice, this means: if the S&P 500 returned 10.0% and VTI returned 9.98%, the difference is 0.02%. If this 0.02% difference repeats year after year with small variance, the tracking error is roughly 0.02%.
Tracking error is distinct from expense ratio. A fund might charge 0.05% in fees but have 0.03% tracking error (meaning it slightly outperforms its expense ratio, perhaps due to securities lending). Or it might charge 0.05% and have 0.07% tracking error (meaning it underperforms its expense ratio due to operational friction).
Why tracking error exists
Several factors create tracking error:
Sampling: Most large index funds don't hold every index constituent. VTI holds roughly 3,500 stocks but the underlying CRSP index includes 3,500-4,000 stocks. The fund samples the index, holding every large-cap stock and a representative sample of smaller caps. This sampling introduces a small performance difference from the true index.
Sampling is actually efficient. Holding every single small-cap stock would require buying hundreds of tiny positions, incurring high trading costs and administrative burden. Holding 90% of the index weight in 1,500 stocks achieves nearly identical returns at much lower cost.
Cash drag: Index funds hold cash to meet redemptions. If the fund is 2-3% cash and the stock market rises 10%, the cash position (earning ~5%) drags returns by ~0.15%. This is a necessary cost of managing redemptions efficiently.
Rebalancing frequency: Index funds rebalance when index changes occur or when sector weights drift too far. Rebalancing triggers selling winners and buying losers—trading that incurs costs. The frequency and timing of rebalancing affects tracking error.
Corporate actions: When companies issue dividends, the fund must decide when to reinvest. A dividend paid on Tuesday might be reinvested the same day or a few days later. This timing creates small tracking error. Similarly, stock splits, mergers, and index reconstitution events require immediate action, and timing differences create small performance gaps.
Securities lending: Some funds lend out securities to short sellers, earning income. This income can offset expense ratios and improve tracking. A fund with 0.05% expense ratio might have 0.03% tracking error because securities lending income covers part of the costs.
Sampling error quantified
A well-designed sampling strategy keeps tracking error to 0.01-0.03%. The sampling approach holds:
- All stocks in the top 1,000 by market cap (covering ~85% of market value)
- A statistical sample of the remaining 2,000+ smaller stocks
The result is that the fund captures 99%+ of the index return while avoiding the friction of trading the smallest, least-liquid securities.
Quality index funds and their tracking error
VTI (Vanguard Total Stock Market ETF): Tracking error approximately 0.01-0.02% annually. The fund holds 3,500+ stocks and its tracking is exceptionally tight. Expense ratio is 0.03%, so the fund delivers nearly 100% of index return minus its stated fees.
VTSAX (Vanguard Total Stock Market Mutual Fund): Nearly identical to VTI with tracking error ~0.01-0.02%. The mutual fund structure has slightly higher administrative overhead, but the difference is negligible.
FSKAX (Fidelity Total Market Index Fund): Tracking error ~0.03-0.05%. Slightly worse than Vanguard's funds, but still excellent. The expense ratio is 0.03%, matching Vanguard.
SCHZ (Schwab U.S. Aggregate Bond ETF): Bond fund tracking error is typically tighter than stock fund tracking error (less rebalancing needed). SCHZ has tracking error ~0.02%.
VEU (Vanguard International Developed Markets ETF): International funds have slightly higher tracking error (~0.05%) than US funds due to currency exchange rates and less liquid foreign markets.
For comparison, poor-quality index funds might have tracking error of 0.10-0.15%, indicating inefficiency or higher costs than disclosed.
How tracking error accumulates
Over a single year, 0.02% tracking error is negligible—a $100,000 investment has a $20 annual performance gap. Over 30 years, this compounds.
If VTI has 0.02% annual tracking error and produces a 9.98% annual return (10% market return minus 0.02% tracking error), compared to a theoretical perfect index fund returning 10%, the 30-year gap is:
- Perfect index: $1,000,000 grows to $13,785,000
- VTI: $1,000,000 grows to $13,585,000
- Difference: $200,000 (1.4%)
This is tiny. The gap between 0.02% and 0.10% tracking error compounds to roughly 5-10% wealth difference over 30 years, more material but still smaller than the impact of expense ratio alone.
Tracking error vs. active management
Here's a critical distinction: tracking error is unavoidable and small in index funds. Active management is avoidable and large.
An actively managed fund trying to beat its index will have "active error" (the difference between its return and the index). This is typically 1-3% annually (the underperformance from failing to generate alpha). This is fundamentally different from an index fund's tracking error.
An active fund might have 0.01% tracking error (perfect tracking) but still underperform the index by 1.5% because the fund is intentionally holding different securities to try to beat the index (and usually fails).
Causes of poor tracking error
If a fund has 0.10%+ tracking error, potential culprits include:
High portfolio turnover: If a fund is claimed to be an index fund but has 20%+ annual turnover, it's probably trading excessively. Index funds should have 2-5% turnover.
Excessive cash drag: If the fund is holding 5%+ cash to meet redemptions, it's inefficient at managing cash flows.
Securities lending practices: Some funds have lenient securities lending programs that interfere with index replication.
Poor fund management: Some fund companies simply run their index funds less efficiently than competitors, resulting in higher tracking error.
Complex index: Some specialized indexes (emerging market small-cap, currency-hedged) are inherently harder to track, and tracking error might be 0.15-0.25%. This doesn't mean the fund is poorly run; it means the index itself is hard to replicate.
Tracking error in different asset classes
Large-cap US stock funds: Tracking error typically 0.01-0.05% because US markets are efficient and liquid.
Small-cap US stock funds: Tracking error typically 0.05-0.15% because smaller stocks are less liquid and harder to sample.
International developed markets: Tracking error typically 0.05-0.10% due to currency exchange, lower liquidity, and operational complexity.
Emerging markets: Tracking error typically 0.10-0.25% due to less liquid markets, capital controls, and trading restrictions.
Bond funds: Tracking error typically 0.02-0.08% because bond markets are large and less volatile than stocks.
Target-date funds: Tracking error typically 0.05-0.15% because they hold multiple underlying funds and dynamically rebalance the glide path.
Does tracking error matter practically?
For most buy-and-hold investors, tracking error in the 0.02-0.05% range is negligible. It's not worth changing funds to improve tracking by 0.02%. The transaction costs of switching often exceed the benefit of better tracking.
However, if comparing two similar funds:
- Fund A: 0.03% expense ratio, 0.02% tracking error
- Fund B: 0.03% expense ratio, 0.10% tracking error
Fund A is clearly superior because it's delivering the index at a lower drag.
If comparing:
- Fund A: 0.03% expense ratio, 0.02% tracking error (total drag 0.05%)
- Fund B: 0.05% expense ratio, 0.03% tracking error (total drag 0.08%)
Fund A is better, though the difference is tiny.
Understanding the index prospectus
When evaluating a fund, the prospectus discloses:
- Target index (what the fund is trying to track)
- Expense ratio (annual fee)
- Portfolio turnover (how much the fund trades)
- Benchmark and comparison statistics
Some prospectuses report "Annualized Tracking Difference," which is the average annual difference between the fund and its benchmark (including expense ratio). This is more accessible than formal "tracking error" (which is the standard deviation).
A tracking difference of -0.05% means the fund underperformed its index by 0.05% on average. This is typical and acceptable.
How index funds improve tracking
The practical threshold
Most investors should look for index funds with:
- Expense ratio under 0.10%
- Tracking error under 0.05%
- Portfolio turnover under 5% annually
- At least $1 billion in assets (sufficient scale to avoid operational friction)
VTI, VTSAX, FSKAX, SCHZ, and similar flagship index funds meet these criteria. Niche index funds (emerging market small-cap, leveraged indices) might have higher tracking error due to inherent complexity, but mainstream index funds are solid.
Related concepts
Next
With chapters on fund selection behind you, the final section is the chapter overview, which ties together the principles of fund selection and prepares you for the next chapter on implementation—how to actually build and maintain a portfolio using the funds and principles covered here.