how indexes are constructed and maintained

In short

A friend of mine once asked me who decides which companies end up in the famous stock lists on the evening news. If you have wondered the same thing, here is the short answer. An index is a list of securities that follows a written rulebook, and a committee or a computer applies that rulebook on a set schedule. The rulebook says which companies qualify, how much weight each one gets, and when the list changes. You can read that rulebook for free on the website of the company that publishes the index. You can also look up the fee on any fund that tracks an index, because the fund's prospectus lists it. Know the rules, and the list stops being a mystery.

The whole of it

What it is

I once watched a neighbor sort apples at a roadside stand, setting the biggest in one crate and the smallest in another. An index works much the same way, only with companies. If you are holding a fund that tracks an index, you own a slice of whatever that rulebook picks. The index itself is not something you can buy. It is a measuring stick, a number built from a list of securities and updated as prices move. A separate fund can copy the list, and that copy is what people actually own.

Every index starts with a universe. That word just means the pool of companies the rulebook is willing to look at. One index may look only at large American companies. Another may look at small ones, or at bonds, or at firms in a single country. The publisher decides the pool before anything else happens.

How it works

A friend of mine who ran a small hardware store used to say that the first job is deciding what belongs on the shelf. Index builders think the same way. After the universe is set, the rulebook adds tests. A company may need a minimum size, measured by its market capitalization, which is the share price times the number of shares held by the public. It may need shares that trade easily, so a fund can buy and sell without moving the price much. It may need to be profitable or to list on certain exchanges. Some publishers hand the final call to a committee, and others let a formula decide.

Then comes weighting, which means how much each company counts. Many well known indexes use market cap weighting. A bigger company gets a bigger slice, and a smaller one gets a smaller slice. Other indexes use equal weighting, where every company counts the same. Some weight by a measure such as dividends. The method changes what the index does, so it is worth knowing which one you own.

Plain upkeep matters too. If you have ever kept a garden, you know it needs weeding. Indexes get tended on a calendar. The publisher reviews the list at set times, often every quarter, and swaps companies in or out when they no longer fit. This is called reconstitution. In between, the publisher may rebalance, which means resetting the weights back to what the rulebook calls for. A company that merges, fails, or gets bought out leaves the list and a replacement steps in.

Here is the part that surprises people. A fund that copies an index has to trade when the index changes. Those trades cost money, through commissions and through the small gap between what buyers pay and sellers accept. The cost shows up in the fund's results, not on a bill. Fewer changes usually mean fewer trades.

The numbers, and where to find yours

I know a woman who kept every statement in a shoebox and never once read the fine print. Her habit cost her, and it is an easy one to fall into. Three numbers deserve your attention, and each one is public.

The first is the fund's expense ratio, which is the yearly fee taken from the fund, shown as a percent of what you hold. You will find it in the fund's prospectus, the official document every fund must publish. The Securities and Exchange Commission keeps these filings on its EDGAR database, and the fund company posts them on its own site too. The second is turnover, which tells you how much of the fund gets traded in a year. It sits in the same prospectus. The third is the index methodology, the rulebook itself, which the publisher posts on its website.

Some limits come from law and change by year. If you hold the fund inside a tax account, the yearly contribution cap for that account is $7,500 for 2026 (source, checked 10/4/2026), and the catch up amount for people at the age set by law is the current figure, which the official source publishes each year. The age when catch up contributions begin is the current figure, which the official source publishes each year. Check those figures on the IRS website before you act, since they are set by law and the site fills in the verified number with its date.

A worked example

Let me tell you about a woman named Marcy, who is forty one and teaches fifth grade. She wants to understand what a fee does over time, so she works it out on paper at her kitchen table. She has $20,000 in a fund that tracks an index. Fund A charges 0.05 percent a year. Fund B charges 0.75 percent a year. For simple math, she leaves out growth and looks at the fee alone for one year.

On Fund A, 0.05 percent of $20,000 is $20,000 times 0.0005, which comes to $10. On Fund B, 0.75 percent of $20,000 is $20,000 times 0.0075, which comes to $150. The gap is $150 minus $10, or $140 in a single year.

Marcy then asks what that gap looks like over time. She picks a made up growth rate of 5 percent a year after fees, only to show the shape of it. Every figure here is her own plain example, not a forecast. Over 20 years, $140 a year does not stay at $140. The money that went to fees can no longer grow for her. At 5 percent, a steady $140 a year for 20 years grows to about $4,630. She gets that by multiplying $140 by the factor 33.07, which is how much a yearly amount grows when it earns 5 percent for 20 years. Then $140 times 33.07 is $4,629.80, and she rounds it.

She sat back and smiled. A small fee, repeated, adds up. Her takeaway is not about which fund to pick. It is that she now knows how to read one line in a prospectus and what that line costs her in dollars. That is a good evening's work.

Where it goes wrong

I once bought a coat because the tag said it was warm. It was not. Names can fool you the same way. Two funds can both say they track large companies and still follow very different rulebooks, so the name alone tells you little. Read the methodology.

People also mix up the index and the fund. The index is the list. The fund is the thing that copies it, and the copy is never perfect. The gap between the two is called tracking error, and fees, trading costs, and timing all feed it. Keep that in mind.

Another trap is weight. In a market cap weighted index, a few giant companies can carry a big share of the whole. If those few have a rough year, the index feels it. Look up the top holdings on the fund's page and see how much the biggest names count.

Then there is the calendar. Because the list changes on a schedule, a company can be added or dropped on a known date. Funds that copy the index must trade around that date. Those trades are part of what you pay for, even though no bill arrives.

One more thing is easy to miss. A rulebook can change. The publisher may adjust its tests or its schedule, and it posts those changes on its own site. A fund you chose years ago may follow a rulebook that has shifted since. Check back now and then.

Questions to answer before you leave this page

Do you know which index your fund tracks, and have you found the publisher's written rulebook for it? Can you say in your own words how that index picks its companies and how it weights them? Have you looked up the expense ratio in the prospectus, and do you know what it costs you each year in dollars on the amount you hold? Do you know the fund's turnover, and what that might mean for trading costs inside the fund? How much of the index sits in its top few companies, and are you comfortable with that? When does the publisher review the list, and what happens to the fund on those dates? And if the rulebook changed tomorrow, where would you go to read about it?