An Index Fund Tries to Follow the Scoreboard

Buy the whole haystack instead of hunting for the needle. That was the pitch when the first retail index fund launched in 1976, and it sounded like surrender until the returns came in.

An index fund is a mutual fund or ETF built to track a chosen market index rather than beat it. Instead of paying a manager to pick winners, it follows the benchmark's list.

How tracking works

The fund holds the index's securities at roughly the index's weights, trading mainly when the index itself changes. Managing by rulebook is cheap, which is why index funds tend to carry low expense ratios.

They still can't match the benchmark perfectly. If the index returns 8% and the fund delivers 7.85%, that 0.15-point gap — the tracking difference — reflects fees, trading costs, cash held for redemptions, and sampling choices.

What you get, and what you give up

One purchase buys broad diversification — hundreds or thousands of securities at once. What you give up is any chance of beating the market the fund tracks.

You also accept the full ride down. A fund that faithfully tracks its index in rallies will track it just as faithfully in crashes.

Two look-alike funds can differ

Two funds both labeled large-cap index may follow different benchmarks with different eligibility rules, rebalancing dates, and dividend treatment. Check the named index, the expense ratio, and the historical tracking difference before calling them interchangeable.

Cap weighting adds its own wrinkle. A fund holding 500 names can still be concentrated, because the largest handful of companies carries a huge share of the weight.

Passive fund, active investor

The fund follows rules, but somebody still picks the index, the stock-bond mix, the account type, and the time horizon. Those decisions do most of the steering, whether the wrapper is a traditional fund or an exchange-traded fund.

The SEC's introduction to investing lays out how index funds fit alongside other choices.

Before buying any index fund, read one line of the prospectus first: the name of the index it promises to follow.