How to Choose an ETF: Expense Ratio, Volume, Tracking Error

The Three Things That Actually Matter

When two ETFs track the same or a similar index, the decision usually comes down to three measurable factors: the expense ratio (what it costs you every year), trading volume and liquidity (how cheaply you can get in and out), and tracking error (how closely it actually follows its stated index). Everything else — marketing, fund size alone, brand name — matters far less than these three.

Expense Ratio: The Cost You Pay Every Year

The expense ratio is the annual fee, expressed as a percentage of assets, deducted automatically from the fund's returns. It's not billed separately — it just quietly reduces your return every year, which is exactly why small differences compound into large ones.

• A 0.03% expense ratio vs. a 0.50% expense ratio on an otherwise identical index fund is a 0.47 percentage point annual drag — over 20 years, that difference alone can amount to a meaningfully different ending balance.
• For broad index ETFs (S&P 500, total market, major bond indices), expense ratios have compressed to near-zero at major providers, so paying meaningfully more for the same exposure is rarely justified.
• For niche, thematic, or actively managed ETFs, higher expense ratios are more common — the question is whether the fund's strategy justifies the extra cost versus a cheaper, broader alternative.

Trading Volume and Liquidity

An ETF's own trading volume affects how cheaply you can buy and sell it, separate from the cost of the fund itself.

• Bid-ask spread: low-volume ETFs tend to have wider spreads between the buy and sell price, meaning you lose a small amount on every round trip just from that gap.
• Average daily volume: higher volume generally means tighter spreads and more reliable execution, especially for larger orders.
• Assets under management (AUM): a very small or new fund carries a higher risk of being shut down or delisted if it never attracts enough assets, which can force you to sell at an inconvenient time.

For a fund you plan to hold for years and rarely trade, a modest spread is a minor cost. For a fund you plan to trade actively, liquidity becomes a much bigger factor in your actual realized returns.

Tracking Error: Does It Actually Do What It Says

Tracking error measures how much an ETF's returns deviate from the index it's designed to follow, over time. A fund with a stated goal of matching the S&P 500 that consistently underperforms the index by more than its expense ratio alone would explain has a tracking error problem — something beyond fees (trading costs, sampling instead of full replication, cash drag) is causing it to fall short of its benchmark.

• Low tracking error means the fund is doing its one job well: delivering the index's return, minus fees.
• Higher tracking error is more common in ETFs holding less liquid underlying assets (certain bond indices, small-cap international stocks, commodities) where perfectly replicating the index is harder.
• You can check tracking error by comparing a fund's stated annual return to the index's return over the same period — a well-run passive fund's gap should be close to its expense ratio, not meaningfully larger.

A Simple Comparison Framework

Factor	What to check	Red flag
Expense ratio	Annual % fee, compare to similar funds	Paying much more than a comparable index fund with no clear reason
Volume / liquidity	Average daily volume, bid-ask spread, AUM	Very low volume, wide spreads, or a small, newly launched fund
Tracking error	Fund return vs. index return over same period	Underperformance well beyond what the expense ratio explains

Other Factors Worth a Quick Check

• Holdings: does the fund actually hold what you think it holds? Two ETFs with similar names can have meaningfully different sector weights or methodologies.
• Tax efficiency: for taxable accounts, how a fund handles capital gains distributions can matter, particularly for actively managed or less tax-efficient strategies.
• Issuer: established providers with a long track record are generally lower-risk in terms of fund survival than newer, smaller issuers, though this is a secondary consideration behind the three core factors above.

What to Ignore

Past performance alone, especially over short windows, tells you little about whether a fund is a good structural choice — two funds tracking the same index will have nearly identical performance, so the decision should be made on cost, liquidity, and tracking fidelity, not on which one happened to be up slightly more last year.

Summary

Choosing between ETFs that offer similar exposure comes down to expense ratio (the direct annual cost), trading volume and liquidity (the cost of getting in and out), and tracking error (whether the fund actually delivers the index's return). A fund that's cheap, liquid, and tracks its index closely will almost always outperform a more expensive, thinly traded, or loosely tracking alternative over the long run, regardless of brand recognition.

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Related reading:

• Sector ETFs Explained: How to Trade Rotation — using sector ETFs once you understand fund selection basics
• Dollar-Cost Averaging vs Lump Sum Investing — how to actually deploy capital into a chosen ETF
• Portfolio Diversification: How Many Stocks Is Enough? — how ETFs fit into a diversified portfolio
• Growth vs Value Investing: Which Wins Long-Term? — a factor distinction relevant to picking thematic ETFs