How Index Funds Work — and Why They Diversify Risk
An index fund is a fund that tries to match a published market index, not beat it. It buys the securities in that index, in roughly the proportions the index sets, and it changes its holdings mainly when the index changes. That single design choice explains most of what follows: broad exposure, low trading, and returns that track the market it follows, minus fees. What an index is, in plain terms An index is a list with rules. It names a group of securities — for example, 500 large U.S. companies — and a method for weighting them, often by market value. You cannot buy an index directly. An index fund is the vehicle that turns that list into something you can own: it holds the securities on the list so your money moves with the group, rather than with one company’s fortunes. How the fund copies the index Some funds buy every security in the index. That approach is called full replication, and it works best when the index is not too large or hard to trade. Other funds use sampling: they ...