Index Construction and its Constituents

A commodity index is a statistical measure that reflects the overall price movement of a selected basket of commodity contracts. It aggregates individual commodity prices into a single number, allowing investors to gauge market trends without tracking each contract separately.

The index is constructed by the exchange or a designated index provider, who selects the underlying contracts, decides on their relative importance (weight), and publishes a methodology that must comply with SEBI guidelines. Because the index is published in real time, it serves as a benchmark for index‑linked products such as ETFs, futures and swaps.

Exam relevance: NISM questions often ask you to identify which component (price, weight, divisor) influences the index level, or to choose the correct weighting method for a given scenario. Typical traps include confusing the index level with the price of a single commodity and overlooking the divisor adjustment.

• Index level – the calculated number representing the basket.
• Underlying contracts – the actual futures or spot contracts that feed price data.

The three core components are:

Underlying contracts – usually front‑month futures because they are the most liquid. Spot prices may be used for commodities that lack an active futures market, but the methodology must state the source.

Weighting scheme – determines how much each contract contributes to the index. Common schemes are price‑weight, quantity‑weight and equal‑weight. The chosen scheme must be disclosed in the index prospectus.

Divisor – a scaling factor that keeps the index level stable when contracts are added, removed or when corporate actions (e.g., contract roll‑over) occur. The divisor is adjusted so that the index does not jump artificially.

• Liquidity filter – many indices require a minimum open interest to qualify a contract.
• Rebalancing frequency – usually monthly or quarterly, as specified by SEBI.

Price‑weighted indices assign a weight proportional to the current market price of each contract. Higher‑priced commodities dominate the index movement, similar to the Dow Jones Industrial Average.

Quantity‑weighted (or volume‑weighted) indices use the number of units (e.g., barrels, ounces) as the weight. This method reflects the physical market size rather than price magnitude.

Equal‑weight gives every selected commodity the same influence, regardless of price or volume. It is useful when the index aims to represent a diversified basket rather than market value.

Exam tip: The NISM syllabus frequently asks you to match a weighting description with its effect on index volatility. Price‑weight leads to higher volatility when a high‑priced commodity moves sharply.

Rebalancing is the periodic process of updating the constituent list and their weights to reflect market realities. SEBI mandates that an index provider disclose the rebalancing schedule—typically monthly or quarterly.

During rebalancing, contracts that no longer meet the liquidity filter are removed, and new contracts may be added. The weights are recalculated based on the latest price or quantity data, and the divisor is adjusted accordingly.

From an exam perspective, you may be asked which of the following actions triggers a divisor change: (a) price movement, (b) contract roll‑over, (c) quarterly rebalancing, or (d) dividend distribution. The correct answer is (b) and (c) because both involve structural changes to the basket.

Because most commodity indices use front‑month futures, the contracts must be rolled over as they approach expiry. The roll‑over replaces the expiring contract with the next‑month contract, which may have a different price level due to contango or backwardation.

The index calculation incorporates the new contract price, and the divisor is adjusted to neutralise the price gap. Failure to adjust the divisor would cause an artificial spike or dip at the roll‑over date.

In the NISM exam, a typical question presents the price of the expiring contract, the price of the new contract, and asks you to compute the adjusted index level. Remember to apply the divisor adjustment formula: New Divisor = (Old Weighted Sum + (NewPrice‑OldPrice)×Weight) ÷ Old Index Level.

SEBI’s "Regulation on Commodity Derivatives" requires that any index used for listed products be approved by the Exchange and disclosed in a prospectus. The methodology, including constituent selection, weighting, rebalancing schedule and divisor adjustment process, must be publicly available.

Index providers must file periodic reports with SEBI, detailing any changes to the basket or methodology. Failure to comply can lead to penalties, and the index may be delisted from the exchange.

For the NISM exam, remember that the regulator’s focus is on transparency and investor protection. Questions may ask which document contains the divisor adjustment policy (answer: the Index Methodology Statement).

Distributors of index‑linked products need to monitor tracking error – the deviation of the product’s return from the index return. High tracking error may arise from transaction costs, imperfect replication, or delayed divisor adjustments.

The expense ratio of the product should be compared with the index’s own cost structure (e.g., data fees). SEBI mandates that the expense ratio be disclosed in the product brochure.

Exam relevance: A typical multiple‑choice question will present two funds tracking the same index, one with higher expense ratio, and ask which will likely underperform over a long horizon. The correct answer is the fund with the higher expense ratio.