NEW YORK, Aug. 25 -- Someone who owns the Nasdaq 100 through a fund is 5.5 per cent below the high water mark. Someone who picked one of the hundred companies inside it is, on the median outcome, 17.6 per cent below theirs. Same index, same twelve months, and twelve percentage points between the wrapper and the contents.

That gap is what index concentration looks like when it is measured rather than asserted. The Nasdaq 100 weights its members by market value, so a small number of very large companies decide what the number on the screen does, and the rest go along for a ride that is not really theirs.

Monday made the point almost too neatly. The index fell 0.79 per cent to 29,077.17 in afternoon trading in New York while its members sp...