How Do You Compare Two Index Funds That Track the Same Benchmark?
Two funds can promise to track the exact same benchmark and still hand investors noticeably different results over time. The gap usually comes down to a handful of quiet details.
The short answer
When funds track the same index, the meaningful differences are usually the expense ratio, how closely the fund actually follows the index (tracking difference), how easily shares trade (liquidity), and whether the fund lends out its holdings for extra income. None of these show up in the headline “tracks the S&P” description, so they require a closer look.
Start with the expense ratio, but don’t stop there
The expense ratio is the most visible cost and the easiest to compare, since it’s stated as a simple annual percentage. A lower expense ratio is a real, compounding advantage over long holding periods. That said, expense ratio alone doesn’t capture the total cost of ownership — trading costs and pricing efficiency matter too, especially for anyone who trades in and out more than occasionally.
Look at tracking difference, not just the stated index
Every fund that follows a benchmark experiences some amount of tracking difference — the gap between the fund’s actual return and the index’s return, driven by fees, trading costs inside the fund, cash drag, and timing of dividend reinvestment. Two funds tracking the identical index can post noticeably different real-world returns over a five or ten-year period purely because of how efficiently each one is run. This is one reason it helps to compare fund performance across multiple time periods rather than relying on a single year’s number.
Consider liquidity and trading costs
For exchange-traded funds, how easily shares change hands matters as much as what’s inside the fund. A fund with thin trading volume can have a wider gap between its buy and sell price, meaning the act of trading itself becomes a cost. Comparing bid-ask spreads across similar ETFs and looking at typical trading volume gives a sense of how expensive it might be to enter or exit a position, separate from the expense ratio.
Check whether the fund lends out securities
Many index funds generate a small amount of extra revenue by lending out the securities they hold to other market participants, such as short sellers, and using that income to offset costs. This practice is common and generally low-risk when managed conservatively, but the specifics — how much lending occurs, how the income is shared with fund holders, and what collateral standards apply — vary by fund family. It’s disclosed in fund literature for those who want to dig in, though it rarely moves the needle enough to be a deciding factor on its own.
Other structural details worth a glance
- Fund size. A very small fund tracking a niche index may face different risks than a well-established one, particularly for cost efficiency and liquidity.
- Sampling versus full replication. Some funds hold every security in the index; others use a representative sample, which can slightly affect tracking accuracy.
- Tax efficiency. Structural differences in how funds handle capital gains distributions can matter more in a taxable account than in a retirement account.
The takeaway
When two funds claim to track the same index, the index itself isn’t where the differences live — the operational details of running the fund are. Comparing expense ratio alongside tracking difference, trading costs, and fund mechanics gives a fuller picture than any single number can on its own.