What “the market” usually means
Every index is a set of decisions about what to include and how to weight it. Those decisions change what the number tells you.
“The market was up today” almost always means one particular index rose. Which one, and how it is built, changes what that sentence is reporting.
Three ways to weight an index
- By market value. Bigger companies count for more. The most common method, and it means a handful of the largest constituents can drive the whole number.
- By price. Higher-priced shares count for more — regardless of company size. An artefact of how some old indices were first built, and it produces genuinely odd results.
- Equally. Every constituent counts the same, which gives a very different picture, weighted towards smaller members.
If the largest few constituents gain enough, the index rises even with a majority of companies down on the day. That is not a flaw — it correctly describes what happened to the total value. It is simply not the same statement as “most shares went up”, and the two get conflated constantly.
What an index is not
It is not a portfolio, a company, or a thing you can buy. It is a calculation published by a provider, with rules about what qualifies and a committee that applies them. Funds exist that track it — see what an ETF is — and those funds are real portfolios with costs.
Inclusion has consequences
When a company is added to a widely tracked index, every fund tracking that index has to buy it. That is a mechanical bid with no opinion about the business attached. Reading it as a verdict on company quality is a mistake, and a common one.
Breadth
Because a headline index can be carried by a few names, analysts look at breadth: how many constituents participated. A rise on narrow breadth and a rise on broad breadth are different events wearing the same number.