Index maintenance

An index has to mean the same thing today as it did yesterday for its percentage change to be meaningful. But the companies inside it keep doing things that change their own share counts and prices — issuing bonus shares, splitting stock, merging with another company. Something has to keep the index comparable through all of that.

The workbook's own words: "Maintenance means adjusting the index for corporate actions like bonus issue, rights issue, stock split, consolidation, mergers etc." It is one of the two processes — alongside index revision — that the workbook distinguishes from index construction itself.

The workbook draws a clean three-way distinction in section 2.5:

The workbook is explicit about what triggers each of the two ongoing processes: "index maintenance issue gets triggered by a corporate action," while index revision is "a continuous exercise" unrelated to any single corporate event. Maintenance is reactive — a bonus issue or a stock split happens, and the index's calculation has to be adjusted to reflect it — while revision is a periodic, ongoing review of whether the index still represents the right set of companies.

Maintenance and revision are both carried out "with the help of various mathematical formulae" by the same specialised agencies that construct the index — Asia Index Pvt Ltd for BSE, NSE Indices Limited for NSE — to keep the index comparable across time despite corporate actions such as stock splits, share issuance, dividends and restructuring events.

A constituent of a broad market index, Kaveri Industries Ltd, announces a 1:1 bonus issue — every shareholder receives one additional share for each share held, doubling the total share count while the underlying business value is unchanged.

Without maintenance. If the index simply used the new (halved) share price against the old share count in its formula, the index would show a sharp, entirely artificial fall — even though nothing about Kaveri's actual business or market value changed.

With maintenance. The index calculation is adjusted for the bonus issue — the corporate action that triggered the maintenance step — so that the index's value continues to reflect Kaveri's true market capitalisation before and after the bonus, with no artificial jump or fall purely from the share count doubling and the price halving.

A rights issue, a stock split, a consolidation of shares, or a merger involving an index constituent would each trigger the same kind of adjustment: the maintenance process exists specifically to absorb these single, identifiable corporate events, one at a time, as they occur — which is different from the ongoing question of whether Kaveri still belongs in the index at all, the question index revision answers.

Chapter 2 (Understanding the Index), section 2.5 (Index management), defines index maintenance in the same breath as index construction and index revision, then returns to it under the "Index Maintenance and Index Revision" heading to contrast what triggers each. Expect a definitional question distinguishing maintenance from revision, and a question on what specifically triggers maintenance (a corporate action) versus what makes revision a continuous exercise.

• Maintenance is triggered by a corporate action; revision is a continuous exercise. This is the single most examinable distinction between the two, and it is easy to reverse under pressure.
• Maintenance keeps the index comparable across time despite corporate actions — it does not change which stocks are in the index. Changing the constituent list is revision's job, not maintenance's.
• The named corporate actions are bonus issue, rights issue, stock split, consolidation and mergers — a specific, workbook-given list, not an open-ended category.
• Maintenance and revision are both done "with the help of various mathematical formulae" by the same specialised index-managing agencies — neither is a manual, ad hoc process.
• Do not confuse index maintenance with index construction — construction is choosing the stocks and methodology in the first place; maintenance is the ongoing adjustment once the index already exists.