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Chapter 14NISM V-DFull chapter

Understanding the Index

In this chapter: What an index is and why it matters as a benchmark and as a derivative underlying · Market-cap weighted, free-float, price-weighted and equal-weighted indices · Index maintenance and the major Indian indices (Sensex, Nifty)

~6 min readLayer 2 · NISM CertificationsFree

An index is a statistical measure of a market or a slice of it — a portfolio of securities expressed as a change from a base value, so the percentage move matters more than the raw number. Indices matter three ways for a derivatives distributor: as a performance benchmark, as a market indicator, and — most relevant to Module 2 — as the underlying for index futures and index options. Understanding how an index is weighted tells you what your derivative is actually tracking. This chapter carries a 5-mark weight.

Foundation

A stock index is a portfolio of securities representing a market or sector, published as a change from a base value. It does three jobs: signals overall (or sector) performance, serves as a benchmark for managed portfolios and mutual funds, and acts as the underlying for index-based derivatives. Indices differ by weighting method. Market-capitalisation weighted: each stock's weight is proportional to its market cap (shares outstanding × price) — bigger companies move the index more. Free-float market-cap weighted: same idea, but only shares actually available for trading (excluding promoter/locked holdings) count — Sensex, Nifty and most global indices use this. Price-weighted: each stock's influence is proportional to its price, not its size (Dow Jones, Nikkei 225 work this way). Equal-weighted: every constituent carries the same weight, which forces periodic rebalancing — selling risers and buying fallers to restore equal weights.

Deep Dive

The weighting method changes what the index tells you. In a market-cap index, a 10% move in the largest constituent swamps a 10% move in the smallest — the index is dominated by giants. In a price-weighted index a ₹3,000 stock sways the index far more than a ₹100 stock even if the ₹100 company is larger, which is why price-weighting is considered a historical quirk rather than best practice. Free-float weighting corrects a real distortion: if a company is 75% promoter-held, its full market cap overstates the shares the market can actually trade, so free-float scales the weight to the tradable portion. India's Sensex and Nifty migrated to free-float precisely for this reason. Equal-weighting sounds "fair" but has a hidden cost — because it rebalances back to equal weights, it mechanically sells winners and buys losers, generating turnover (and, in a fund tracking it, transaction costs and possible tax).

Advanced

For a derivatives distributor the key insight is that an index future or option inherits the index's weighting behaviour. Hedging a large-cap-heavy portfolio with Nifty futures works well because both are free-float market-cap weighted and dominated by the same giants; hedging a broad small-cap portfolio with Nifty futures leaves a large "basis" gap because the underlyings diverge. When a fund claims to be "market-neutral" by shorting index futures, the neutrality only holds to the extent the long book resembles the index — a mismatch in composition is residual, un-hedged risk. This is why understanding index construction is not academic: it is the difference between a hedge that works and one that quietly leaves the investor exposed.

Regulatory references
  • SEBI norms on benchmarking of mutual fund schemes (Total Return Index)
  • Index provider methodology documents (NSE Indices, Asia Index / BSE)
Common mistakes & pitfalls
  • Quoting an "index return" without stating the weighting method — the same stocks give wildly different returns depending on it.
  • Hedging a small/mid-cap portfolio with a large-cap index future and assuming the hedge is complete.
  • Confusing free-float market cap (tradable) with full market cap (all shares including locked promoter holdings).
  • Comparing a fund to a convenient index (Nifty 50) rather than its mandated benchmark.

Frequently asked

Why did Sensex and Nifty move to free-float weighting?
Full market cap over-weights companies with large promoter/locked holdings that are not actually available to trade. Free-float scales each stock's weight to the shares the market can trade, giving a truer picture of investable performance.
Is an equal-weighted index better because it is "fair"?
Not automatically. Equal weighting reduces concentration in giants but forces regular rebalancing (sell winners, buy losers), which raises turnover, cost and tax in any fund that tracks it.

Practice questions

Click each question to reveal the answer and explanation.

Q 1
Using the NISM five-stock data, the market-cap of the index rises from ₹18,800 lakh to ₹42,500 lakh against a base value of 100. The current market-cap-weighted index value is closest to:
  1. (a)104.00
  2. (b)126.06
  3. (c)226.06
  4. (d)242.50
Correct: (c) 226.06
Index = (current market cap / base market cap) × base value = (42,500 / 18,800) × 100 = 226.06. (The rise is 126.06%, but the index VALUE is 226.06 — a classic trap between the level and the percentage change.)
Q 2
For the same five stocks, the price-weighted index goes from a base of 250 to 510. A candidate reports the index "rose to 104." What did they get wrong?
  1. (a)Nothing — 104 is correct
  2. (b)They reported the percentage change (104%) as if it were the index value (510)
  3. (c)They used market-cap weighting by mistake
  4. (d)The base should have been 100, not 250
Correct: (b) They reported the percentage change (104%) as if it were the index value (510)
The price-weighted index VALUE moved from 250 to 510; the percentage CHANGE is (510−250)/250 = 104%. Reporting "104" confuses the % change with the index level. Level vs change is the most common index-question trap.
Q 3
Which pair of indices is price-weighted rather than market-cap weighted?
  1. (a)Sensex and Nifty
  2. (b)Dow Jones Industrial Average and Nikkei 225
  3. (c)Nifty and SX40
  4. (d)S&P 500 and Nifty Midcap 150
Correct: (b) Dow Jones Industrial Average and Nikkei 225
The Dow Jones Industrial Average and the Nikkei 225 are price-weighted — a stock's influence tracks its price, not its size. Sensex, Nifty, SX40 and the S&P 500 are (free-float) market-cap weighted.
Q 4
A fund shorts Nifty futures against a long book of broad small-cap stocks and calls itself "market-neutral." The main flaw is:
  1. (a)Futures cannot be used to hedge at all
  2. (b)The long book and the Nifty are differently composed, so significant residual risk remains
  3. (c)Short futures add leverage but no hedging
  4. (d)Small caps have no market risk to hedge
Correct: (b) The long book and the Nifty are differently composed, so significant residual risk remains
Neutrality only holds to the extent the long book resembles the index being shorted. A small-cap long book vs a large-cap index (Nifty) leaves a large composition gap — real, un-hedged residual risk. Index construction directly determines hedge quality.
Educational purposes only. The numbers, returns, and examples used in this lesson are illustrative. Past performance does not guarantee future results. Mutual fund and securities investments are subject to market risks. This lesson is not investment advice; for advice tailored to your circumstances, consult a SEBI-registered Investment Adviser. Read our full disclaimer.