An index is a number, not a product: who is in × how much they count.
- Size-weighted: a few giants ARE the number
- Funds track it — for a fee, imperfectly
- Every index is a selection, and selections fall too
What this is about
The next lesson says most people don't beat "the index", and that sentence is useless if the index is a vague blur. So first: an index is a number, not a product. It is a formula somebody maintains — a rule for which companies are in, and a recipe for turning their prices into one figure. Nobody can buy an index, any more than they can buy the average temperature.
The two decisions inside every index
Who is in. An index committee or a rulebook decides membership: the largest US companies, everything on one exchange, one sector, one country. Membership changes — companies are added and dropped on schedule — so "the index" of ten years ago is not the same list of names.
How much each one counts. Most famous indexes are weighted by company size: the bigger the company, the more its price moves the number. The practical consequence surprises people every time — in a size-weighted index of 500 companies, a handful of giants can outweigh hundreds of the smaller members combined. "The index rose today" often means "a few very large companies rose today", and the heatmap makes that visible at a glance.
A price-weighted index (a share at 300 counts triple one at 100, regardless of company size) is an older, stranger recipe that survives in one or two famous names. Same companies, different recipe, different number.
The number versus the thing you can buy
Since an index is a formula, tracking it costs money to do — and that is what index funds and ETFs are for: products that hold the members in the index weights so their value follows the number. The lesson What an ETF is covers the wrapper. Three gaps between number and product are worth knowing: the product charges an annual fee, it cannot always match the recipe perfectly (tracking difference), and its price is quoted per unit — the index level and the fund price are different numbers moving together.
A worked example
Build one from scratch. Three fictional companies: Alder worth 800m, Birch worth 150m, Cedar worth 50m — total 1,000m. A size-weighted index gives Alder 80% of the weight, Birch 15%, Cedar 5%. Start the index at a level of 1,000 for convenience. Cedar has a spectacular day and rises 20%; Alder drifts down 2%. The index: 80% × (−2) + 5% × (+20) = −1.6 + 1.0 = −0.6%. The index falls on the day the small company soared, because the recipe says the giant is most of the number.
Now the same three companies equal-weighted — a third each: (−2 + 0 + 20) ÷ 3 = +6%. Same market, same day, one index down and one up six — nothing moved but the recipe. When two headlines disagree about how "the market" did, this is usually why, and it is also why knowing which index a fund tracks matters more than the word "index" on the label.
What this does not tell you
It does not tell you the index is the market. Every index is a selection. The famous ones cover large companies well and everything else barely.
It does not tell you index membership is neutral. Being added to a major index moves a share's price, because tracking money must buy it. The recipe affects the ingredients.
It does not make "the index" a safety claim. The next lesson uses the index as a measuring stick, and a measuring stick can fall 40%. Owning everything diversifies away single-company risk — market risk stays whole.
Where to see this in the app
The Heatmap is this lesson drawn live: every tile a company, tile size = company size — one glance shows how much of the board a handful of giants occupy, which is exactly why they move the number. The page's own footnote says the boards use our large-cap selection rather than a licensed index membership; after this lesson you know precisely what that sentence means.