Active share: The metric that separates real bets from closet indexing

Active share is a simple number: the percentage of a mutual fund's holdings that differ from its benchmark. A fund with 80% active share has 80% of its portfolio in stocks not in the index. The metric was first defined by Cremers and Petajisto in 2009, and it became a quick shorthand for identifying closet indexers. If a fund charges high fees with an active share of 30%, you are paying for active management and getting index-like exposure.

The problem is that many investors treat active share as a standalone quality signal. High active share, the thinking goes, means the manager is making big bets and should outperform. The data does not support that. Research from the original authors later showed that active share alone does not predict future returns. A fund can have 90% active share because it is concentrated in a few stocks that happen to be in the index. It is active at the stock level but not at the sector level.

The better read pairs active share with tracking error, also called active risk. Tracking error measures how much the fund's returns deviate from the benchmark. A high active share with low tracking error suggests the manager's bets cancel each other out in the return stream. That combination often signals a closet indexer who just rotates names within the same sectors. Genuinely active managers have both high active share and high tracking error. Their bets are concentrated in sectors or factors that drive meaningful performance differences.