Active Share Monitor

An Active Share Monitor maximizes portfolio performance by identifying “closet indexers,” optimizing manager fees, and tracking stylistic drifting from a fund’s target benchmark. First introduced by Yale researchers Martijn Cremers and Antti Petajisto, Active Share measures the exact percentage of an equity portfolio’s stock selections and weightings that differ from its benchmark. Implementing a continuous monitor ensures you pay active fees exclusively for true high-conviction management rather than masked passive indexing. Technical Breakdown and Formula

Active Share scales from 0% (a pure passive index tracking fund) to 100% (a portfolio with completely zero overlap against the benchmark).

Active Share FormulaThe metric mathematically sums the absolute differences between the portfolio weights and benchmark weights across all assets, divided by two:

Active Share=12∑i=1N|wportfolio,i−wbenchmark,i|Active Share equals one-half sum from i equals 1 to cap N of the absolute value of w sub portfolio comma i end-sub minus w sub benchmark comma i end-sub end-absolute-value wportfolio,iw sub portfolio comma i end-sub : The weight of asset i within the actively managed fund. wbenchmark,iw sub benchmark comma i end-sub : The weight of asset i inside the designated index.

N: The total universe of unique securities across both portfolios.

Alternatively, it can be viewed as Active Share in Mutual Funds | Touchstone Investments:

Active Share=100%−Overlapping WeightsActive Share equals 100 % minus Overlapping Weights Core Functions of an Active Share Monitor

Using an automated monitor allows institutions and retail investors to implement precise oversight across four key areas: Why Should We Care about Active Share? (Summary)