> sensible thing to do is to create a new index with the new rules
It depends. Indices aren’t funds. They aren’t meant to balance investor interests. They’re meant to communicate some metric about the market.
The S&P tells you how big companies are doing in an index optimized to balance representation against trading cost. So in 2005, float was taken into account for weighting (versus just market cap). This made sense. Also, since the start, the S&P 500 has been a committee-based index. Not rule based. This has made it successful; if you want stable and unchanging, you never went for the S&P 500.
It depends. Indices aren’t funds. They aren’t meant to balance investor interests. They’re meant to communicate some metric about the market.
The S&P tells you how big companies are doing in an index optimized to balance representation against trading cost. So in 2005, float was taken into account for weighting (versus just market cap). This made sense. Also, since the start, the S&P 500 has been a committee-based index. Not rule based. This has made it successful; if you want stable and unchanging, you never went for the S&P 500.