How does liquidity matter for market indexes?
At one level a market index is used as a pure economic time-series. Liquidity affects this application via the problem of non-trading. If some securities in an index fail to trade today, then the level of the market index obtained reflects the valuation of the macroeconomy today (via securities which traded today), but is contaminated with the valuation of the macroeconomy yesterday (via securities which traded yesterday). This is the problem of stale prices. By this reasoning, securities with a high trading intensity are best-suited for inclusion into a market index. As we go closer to applications of market indexes in the indexation industry (such as index funds, or sector-level active management, or index derivatives), the market index is not just an economic time-series, but a portfolio which is traded. The key difficulty faced here is again liquidity, or the transactions costs faced in buying or selling the entire index as a portfolio.
