Index inclusion effect

An index is not a fixed list. Companies that fall below the entry criteria are dropped, and new ones that meet them are added.

For a fund that copies the index, that is not a piece of news. It is an instruction. The dropped stock has to go, completely. The new stock has to be bought, at its full index weight, on the effective date.

Now think about how much money is doing this at once. If passive funds tracking the index are very large, they are all placing the same orders on the same day. The buying pushes the entrant's price up. The selling pushes the leaver's price down.

That is the index inclusion effect. The fund pays for it three times over: in the price it gets, in the brokerage it spends, and in the tracking error it reports.

Why an index is rebalanced at all (section 18.4.3). Companies falling below the threshold are excluded and new companies meeting the criteria are included — but not abruptly, in a calibrated manner, so that the change reflects a genuine shift in the market and fundamental behaviour of the stock. Other reasons are corporate actions such as merger and demerger, suspension of trading, a change in category (for instance inclusion in or exclusion from the derivatives segment), and regulatory action. The premise is to keep the index relevant in the larger interest of investors.

The effect itself. Whenever a rebalance results in the inclusion or exclusion of a constituent, passive fund managers doing full replication completely exit one constituent and buy the new one according to its weight. The workbook names three consequences: an impact on the portfolio, on the transaction cost, and a contribution to the tracking error.

The sibling effect: flows. The same section adds that a passive manager must invest inflows in the same proportions as the benchmark constituents, so large-weighted stocks are bought in large quantities, giving a further push to price and valuations. The reverse happens on outflows. In the workbook's words, passive fund flows can influence the prices of securities in the market — and such price imbalances are often exploited by active managers, which acts as an automatic counterbalancing force.

No figure is given. Section 18.4.3 puts no number on the effect: no percentage price impact, no announcement-to-effective-date window, no cap. The relevant quantities sit one section earlier, in 18.4.1, which is what makes the effect matter — Indian passive AUM of $24bn across 86 products as of March 2020, up from $1bn across 26 products a decade before. The workbook's own condition is that the effect bites when the sheer size of passive funds is large.

Illustrative figures. A fully replicating portfolio of Rs 800 crore tracks a large-cap index. The index provider announces that Stock L leaves and Stock N enters, both at a 0.9% weight, effective the last Friday of the month.

The manager's forced trades on that one day:

Now assume every passive fund on this index is doing the same, and the buying lifts Stock N by 2% above where it traded before the announcement. Our manager's Rs 7.2 crore purchase costs about Rs 14,40,000 more than the pre-announcement price. The forced sale of Stock L, into the same crowd of sellers, gives up a similar amount.

On Rs 800 crore, roughly Rs 25 lakh of avoidable cost is about 0.03% of the portfolio in a single rebalance — a direct subtraction from a portfolio whose entire purpose is to match the index.

An active manager who anticipated the inclusion and bought Stock N a fortnight earlier sold into that demand. The workbook's point exactly: the active side supplies the counterbalancing force.

Chapter 18, section 18.4.3 (Rebalancing of Index), names the index inclusion effect in those words and lists its three consequences — portfolio impact, transaction cost, tracking error. The surrounding text on inflows and outflows is examined alongside it.

Expect a question on which type of passive manager is most affected (a full replicator), on what the effect contributes to (tracking error), or on the reasons an index is rebalanced — where change in category, such as inclusion in or exclusion from the derivatives segment, is the option candidates do not expect.

• This is the effect on prices and portfolios, not the rebalancing process itself. Index revision and index maintenance are how the provider makes the change; the inclusion effect is what happens to everyone who has to follow it.
• Full replicators are hit hardest. A sampling portfolio may not hold the leaver at all, so it may have nothing to sell.
• It cuts both ways. Exclusion forces a complete exit, which is a price impact in the opposite direction. Reading the term as only about additions loses half the marks.
• It creates tracking error, not alpha. The passive manager is not trying to profit from it; the active manager on the other side is.
• Index rebalancing is deliberately calibrated. The workbook stresses that constituents are not changed abruptly, because the point is to reflect a genuine change in market and fundamental behaviour.