WSJ: How Important is "Active Share" for Fund Managers?

By SIMON CONSTABLE

Just how active is the manager of your actively managed mutual fund? And how much
does it matter?

A debate is on over the concept of “active share”—a measure of how much a portfolio’s
stocks differ from those in its benchmark. The issue is whether it is a valid way to
evaluate managers.

It all started in a 2006 working paper and 2009 article by Martijn Cremers and Antti
Petajisto, then professors at the Yale School of Management. They suggested ranking
funds from zero to 100% based on how much their holdings diverge from a benchmark
index. An active share of 60%, for example, means that 40% of the fund’s stocks merely
match what is in an index—a mix that would make the fund’s manager a “closet indexer,” the professors wrote. Read more here.