Active Share
Active share is a measure of how much a portfolio's holdings differ from the holdings of its benchmark. It runs from 0% to 100%, where 0% means the portfolio matches the benchmark exactly and 100% means the portfolio shares no holdings with the benchmark at all.
Active share looks at the portfolio's positions rather than its returns, which sets it apart from most active-management statistics. Two funds can post the same benchmark-relative return yet differ sharply in how far their holdings stray from the benchmark, and active share captures exactly that distance. The measure was introduced by Martijn Cremers and Antti Petajisto in 2009 as a way to quantify how genuinely active a fund is, separating funds that take real positions away from the benchmark from those that stay close to it while charging active-management fees.
Definition
Active share measures the fraction of a portfolio's holdings that differs from its benchmark. It is calculated by adding up the absolute differences between each position's weight in the portfolio and its weight in the benchmark, then dividing by two so the result falls between 0% and 100%. The division by two prevents double-counting, since every overweight in one holding is offset by an underweight somewhere else.
A low active share means the portfolio's weights sit close to the benchmark's weights, so the portfolio behaves much like the index it tracks. A high active share means the weights depart substantially, through holdings the benchmark excludes, much larger or smaller positions in shared holdings, or both. The figure says nothing on its own about whether those departures helped or hurt; it only quantifies how different the portfolio is.
Key Principle
Active share describes holdings, not outcomes. A portfolio with high active share has taken positions that differ markedly from its benchmark, which gives it the capacity to produce a different result, better or worse, than the benchmark. The measure quantifies the opportunity for active decisions to matter, not the quality of those decisions, so a high active share is a precondition for benchmark-relative performance rather than a predictor of its direction.
How It Works
Active share rises as a portfolio's weights move away from the benchmark's weights. A fund can increase its active share by holding securities outside the benchmark, by overweighting or underweighting securities the benchmark includes, or by leaving out benchmark members entirely. Each of these choices widens the gap between the two sets of weights, and the sum of those gaps, halved, is the active share.
Active share works alongside tracking error (a measure of how much a portfolio's return varies from its benchmark's return over time), and the two together describe different facets of being active. Cremers and Petajisto argued that a fund can have high active share but low tracking error if its distinctive holdings are spread across many names that tend to offset one another, or high tracking error from concentrated factor and sector bets even without unusual stock-level holdings. Reading the two measures together gives a fuller picture than either alone, since one looks at positions and the other at return variability.
| Feature | Active Share | Tracking Error |
|---|---|---|
| What it looks at | The portfolio's holdings and weights | The portfolio's return over time |
| What it captures | How different the positions are from the benchmark | How much the return varies from the benchmark |
| Driven mainly by | Stock-level selection away from the index | Factor, sector, and systematic bets |
| Range | 0% to 100% | Open-ended, expressed as a return variability |
Interpretation
A high active share signals that a fund is genuinely active and therefore capable of differing from its benchmark, which is the original motivation Cremers and Petajisto gave for the measure. It helps identify funds that stay close to the index while charging active fees, a pattern sometimes called closet indexing. For a fee-conscious investor, comparing active share against the cost of active management clarifies how much active management is actually being delivered.
High active share does not establish that a fund will perform well, and that distinction matters. Departing from the benchmark creates the chance to outperform and the equal chance to underperform, since a distinctive portfolio can be distinctively wrong. Active share is most usefully read as a measure of activeness and of the potential for active return, not as a forecast, and it is most informative when paired with an analysis of where the active positions come from and how much they cost to maintain.
Known Limitations
Limitations to Keep in Mind
- Measures difference, not skill. A high active share shows only that holdings depart from the benchmark, not that the departures are well chosen. A distinctive portfolio can differ from the index in ways that help or hurt, and the measure cannot tell which.
- Sensitive to the benchmark chosen. Active share depends on which benchmark a fund is compared against. Measuring the same fund against a different or poorly matched index can raise or lower its active share without any change in the portfolio itself.
- Says nothing about risk taken. The measure ignores how much risk the active positions carry. A fund can post high active share through many small, offsetting deviations or through a few concentrated bets, and active share alone does not distinguish the two.
- Can be raised mechanically. A manager can lift active share by adding off-benchmark holdings or extreme weights without improving the underlying process. A higher number is not, by itself, evidence of a more thoughtful strategy.
- Evidence on prediction is debated. The original research linked higher active share to stronger benchmark-relative results, but later studies questioned how much of that relationship holds once benchmark choice and other factors are controlled. The measure remains useful for describing activeness while its value as a predictor is contested.
Academic Origin
Active share was introduced by Martijn Cremers and Antti Petajisto in their 2009 paper "How Active Is Your Fund Manager? A New Measure That Predicts Performance." The researchers proposed active share as a holdings-based complement to tracking error, arguing that the two measures together separate stock selection from factor and sector bets. Their work gave investors a way to identify closet indexing and to ask whether a fund's fees matched the amount of active management it provided.
The measure sits within the broader literature on active versus passive management and connects to ideas such as factor investing and smart beta, which describe rules-based strategies that depart from a standard index in systematic rather than discretionary ways. Active share helped sharpen the question of what counts as active, and subsequent research has continued to examine how reliably it relates to results, making it a measure that is widely cited and actively debated.
Further Reading
- Cremers, K.J.M. and Petajisto, A. (2009). "How Active Is Your Fund Manager? A New Measure That Predicts Performance." The Review of Financial Studies, 22(9), 3329–3365.
- Petajisto, A. (2013). "Active Share and Mutual Fund Performance." Financial Analysts Journal, 69(4), 73–93.
- Grinold, R.C. and Kahn, R.N. (2000). Active Portfolio Management: A Quantitative Approach for Producing Superior Returns and Controlling Risk. 2nd ed. McGraw-Hill.
Related Terms
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