How Index Funds Actually Work
Tracking an index sounds mechanical. The decisions inside it are where funds differ.
An index fund aims to match the return of a defined index rather than beat it. The simplicity of that description conceals a set of choices that make otherwise similar funds behave differently.
The index is a product
An index is constructed by a company according to published rules: which securities qualify, how they are weighted, and when the list is revised.
Those rules are decisions. A broad market index and a large-company index of the same country will not perform identically, and two indices with similar names from different providers can hold noticeably different things. Read what an index actually contains before assuming what it represents.
Full replication or sampling
A fund can hold every constituent in proportion, or hold a representative sample designed to behave like the index. Sampling is common where an index contains thousands of illiquid securities, and it introduces a small amount of divergence.
Tracking difference
The measure that matters is how closely the fund's return follows the index after costs. Fees explain most of the gap; trading costs and the treatment of dividends explain the rest.
Compare funds on tracking difference over several years rather than on headline fee alone. A slightly dearer fund that tracks tightly can beat a cheaper one that does not.
Rebalancing
When an index changes its constituents, tracking funds must trade. Because the changes are announced in advance and every tracking fund must act, this is a well-known effect that other market participants trade around. It is one of the small, structural costs of indexing.
What indexing does not do
An index fund gives you the market's return, including its declines. It provides diversification within its index and nothing beyond it — a fund tracking one country's large companies is concentrated by any global measure, however many holdings it contains.
Knowing precisely what your index covers is the whole of the risk analysis.
This is general information, not financial advice. Nothing here is a recommendation to buy or sell anything; speak to a licensed adviser about your own position.