Understanding the index
How a stock index is built and maintained, free-float weighting, impact cost, and why indices matter for derivatives…
Index futures and options are the most traded contracts in India, and every long-short SIF strategy uses them to build or hedge exposure. If you do not understand how the Nifty 50 is computed, you cannot explain to a client what a short index position actually does. This chapter builds that understanding with a hand-worked index.
What an index is
A stock index is a single number that tracks the combined performance of a basket of shares chosen to represent a market or a segment. Nifty 50 represents the 50 largest, most liquid stocks on NSE; Sensex tracks 30 on BSE. Sector indices (Bank, IT, Pharma), size indices (Midcap 150, Smallcap 250) and thematic indices follow the same logic. An index is also a benchmark for funds, the underlying for futures and options, the target for index funds and ETFs, and a barometer of sentiment.
Nifty 50 has a base date of 3 November 1995 and a base value of 1,000. The index level on any day is the current market capitalisation of its constituents divided by the base market capitalisation, multiplied by the base value. The base is adjusted whenever constituents change or corporate actions occur, so that the index itself does not jump for reasons unrelated to price.
Methods of construction
- Price-weighted. The index is the average of constituent prices; a high-priced share dominates whatever its company size. The Dow Jones Industrial Average works this way. Rare in India.
- Market capitalisation-weighted. Each stock's weight is its market value (price × shares outstanding) as a share of the total. Bigger companies move the index more.
- Free-float market capitalisation-weighted. The same, but only counting shares actually available for trading: promoter holdings, government stakes, strategic holdings and locked-in shares are excluded. Nifty 50 and Sensex both use this method, so a large company with a 75% promoter stake has a smaller index weight than its full market cap suggests.
- Equal-weighted. Every stock has the same weight at each rebalance, which gives smaller constituents more influence.
| Stock A: price ₹500 × 10 crore free-float shares | ₹5,000 crore |
| Stock B: price ₹200 × 20 crore free-float shares | ₹4,000 crore |
| Stock C: price ₹1,000 × 1 crore free-float shares | ₹1,000 crore |
| Total free-float market cap | ₹10,000 crore |
| Base market cap (base value 1,000) | ₹2,000 crore |
| Index = 10,000 ÷ 2,000 × 1,000 | 5,000 |
| Stock A rises 10% to ₹550; total becomes ₹10,500 crore | |
| New index = 10,500 ÷ 2,000 × 1,000 | 5,250 (up 5%) |
Stock A has a 50% weight, so its 10% rise lifts the index by 5%. Stock C has the highest price but only a 10% weight; a 10% rise in C moves the index just 1%. That is the difference between price weighting and free-float market cap weighting, and the exam likes to test it with exactly this sort of table.
Impact cost: the liquidity test
An index of illiquid stocks would be useless as an underlying for derivatives, because nobody could arbitrage it. NSE therefore screens constituents by impact cost, the percentage mark-up a trader actually pays over the ideal mid-price when executing an order of a given size. The ideal price is the midpoint of the best bid and best ask; the actual price is the average at which the whole order would be filled by walking through the order book.
Suppose the best bid is ₹99.50 and the best ask is ₹100.50, so the ideal price is ₹100.00. A buy order large enough to clear several price levels fills at an average of ₹100.30. Impact cost = (100.30 − 100.00) ÷ 100.00 = 0.30%. A stock enters the Nifty 50 only if its average impact cost for the specified order size stays within the index provider's limit (0.50% for an order size the provider notifies), measured over six months on most trading days.
Maintenance, rebalancing and corporate actions
An index committee reviews constituents periodically; Nifty 50 is rebalanced semi-annually, with changes announced in advance so that index funds and ETFs can adjust. Stocks are added or dropped based on free-float market cap, liquidity, listing history and trading frequency. Corporate actions are handled by adjusting the base market cap: a bonus or split changes the share count but not the value, so the index is unaffected; a rights issue or a change in free float changes the base; a dividend is not adjusted in a price index, which is why a total return index (which reinvests dividends) always outperforms the price index over time. Since 2018 SEBI requires mutual funds to benchmark against total return indices.
Why the index matters for an SIF
An Equity Long-Short strategy that wants to reduce market risk without selling its stocks sells Nifty futures; the amount it sells depends on the portfolio's beta to the index, which Chapter 8 computes. A Sector Rotation strategy may hold long positions in one sector index and short futures on another. The quality of those hedges depends on how closely the portfolio tracks the index; the gap is basis risk and tracking error. Knowing what sits inside the index, and how its weights shift, is part of explaining what the strategy is really doing.
Remember
- An index tracks a representative basket; Nifty 50 has base date 3 November 1995 and base value 1,000.
- Nifty 50 and Sensex are free-float market cap-weighted: only shares available for trading count, so promoter stakes are excluded.
- In a market cap index, influence comes from weight, not price; a stock with 50% weight rising 10% moves the index 5%.
- Impact cost = (actual price − ideal mid-price) ÷ ideal price; it measures liquidity and screens index constituents.
- Bonus issues and splits do not move the index because the base market cap is adjusted; dividends are captured only in the total return index.
- SIF long-short strategies hedge and short through index futures, so basis risk between portfolio and index is a live concern.