Charta is on Google Play: practise on the go. Get the app
Charta

Set out the choices made when building an index

CFA® Level I · Equities · Equity Issuance and Trading · EQ.3.b

The idea

An index provider must decide what the index is intended to measure, which determines the target market; how constituents are selected, whether by rule or by committee; how they are weighted; and how often the index is rebalanced and reconstituted. Each decision has consequences: a narrow selection makes the index unrepresentative, a committee introduces judgement and discretion, and the weighting scheme determines both the risk profile and the turnover an index fund must incur. An index is not a neutral observation of a market but a set of choices with a return attached.

A practice question

From the question bank, with the reason each wrong option is wrong.

What is the first decision in constructing a security market index?

  1. The rebalancing frequencyWhy not: A maintenance rule, and one that cannot be set until it is known what the index holds and why.
  2. What the index is to measure - correct
  3. The weighting schemeWhy not: Weighting answers how much of each constituent to hold, which presupposes a decision about which constituents belong.

A security market index is a basket of securities used to measure a market. The first decision is the target market: what the index is meant to measure, for example large company shares in one country or investment-grade bonds worldwide. Only then can you choose which securities to include, how to weight them (price, capitalisation or equal) and how to maintain them (rebalancing). A provider cannot choose between capitalisation and equal weighting until it knows which market it is representing.

A harder one

Two providers publish indexes of the same market with different returns. What most likely explains it?

  1. One has made a calculation errorWhy not: Reaches for a mistake when a routine explanation is available: two providers made different, defensible design choices.
  2. One is measured in a different currencyWhy not: Possible, but it would be stated. Selection and weighting differences produce return gaps between indexes quoted in the same currency.
  3. They made different selection and weighting choices - correct

An index is not a neutral photograph of a market. The provider makes design choices: which securities qualify (selection), how much each one counts (weighting), and when the list is refreshed. Two providers can make different, reasonable choices for the same market, so their returns differ. For example, an equal-weighted index gives smaller companies more weight than a capitalisation-weighted one, so over a year its return can be quite different. A calculation error is a less likely explanation, and a currency difference would normally be stated.

The hardest tier

Sofia Lindqvist runs an index fund that replicates the published rules of the Northern Mid-Cap Index, whose members are chosen by a selection committee. Over a year her fund lags the index by 0.8% because the committee added two firms that did not meet the published size threshold and removed one that did, with no advance notice. What is the main practical drawback of committee-based selection that this illustrates?

  1. Corporate actions are handled less well by committeesWhy not: Corporate actions are dealt with by the maintenance rules whoever picks the constituents, so they are not the drawback shown here.
  2. Discretion makes membership changes hard to anticipate - correct
  3. Index levels are published less often under committeesWhy not: Index levels are published as often under a committee as under a rule. The problem is unpredictable membership, not the speed of calculation.

An index provider chooses members either by a fixed rule or by a committee. A rule can be written down and modelled in advance, so a fund can copy it exactly. A committee uses judgement, so changes can happen for reasons an investor cannot predict. Here Sofia followed the published size rule, but the committee added two firms that failed it and dropped one that passed, with no notice. Her fund missed these changes and lagged by 0.8%. That unpredictability is the practical cost for anyone tracking the index. Calculation frequency and corporate actions are the same under either method.

In the app: 9 practice questions on this outcome, a narrated walkthrough with diagrams, spaced review that brings it back before you forget it.

Take the free diagnostic →

More in Equity Issuance and Trading