{ "id": "1dc242f3-e287-5065-a59e-bac81c882606", "slug": "active-share", "term": "Active Share", "aliases": [], "category": "Equities", "category_slug": "equities", "difficulty": "intermediate", "definition": "Active Share is a metric that measures the percentage of a portfolio that differs from its benchmark index, calculated as one-half the sum of the absolute differences between each security's portfolio weight and benchmark weight, ranging from 0% (perfect index replication) to 100% (no overlap with the benchmark). It was introduced by Cremers and Petajisto (2009) to distinguish genuinely active fund management from 'closet indexing.'", "key_takeaways": [ "Active Share = 0.5 × Σ |w_portfolio(i) - w_benchmark(i)|; values above 80% are generally considered 'highly active.'", "Active Share is a position-based measure, distinct from tracking error, which is a return-based measure of active risk; a fund can have high active share but low tracking error if its positions are diversified.", "The Cremers-Petajisto research found that high-active-share, low-tracking-error funds ('concentrated stock pickers') outperformed net of fees, while closet indexers underperformed.", "Active Share alone does not predict performance—a manager can have 95% active share and consistently underperform by holding idiosyncratic losers.", "Regulators in several jurisdictions (UK FCA, Netherlands AFM) have incorporated active share monitoring into their oversight of actively managed funds to protect investors from paying active management fees for index-like returns." ], "detailed_explanation": "Active Share provides a quantitative answer to the question 'how different is this portfolio from its benchmark?' The formula sums the absolute differences in weights across all securities and divides by two to avoid double-counting (a position that is overweight in the portfolio is simultaneously underweight in the benchmark by the same amount). A fund with 60% active share holds 60 cents of every dollar in positions that differ from the benchmark—either securities not in the index or overweights/underweights of index constituents.\n\nThe conceptual distinction between active share and tracking error is critical for portfolio analysis. Tracking error measures the standard deviation of the return difference between the portfolio and the benchmark—it is a risk-based, return-space measure that captures how volatile the active bets are. Active share operates in weight-space and captures the structural divergence from the index. A concentrated fund owning 30 large-cap stocks that are also in the S&P 500 (but in very different proportions) can have high active share but moderate tracking error if the stocks are highly correlated. Conversely, a fund that makes modest allocation tilts across many sectors can have low active share but elevated tracking error if those tilts are in volatile sectors.\n\nCremers and Petajisto's original research categorized funds into four quadrants: (1) diversified stock pickers (high AS, low TE), (2) concentrated stock pickers (high AS, high TE), (3) factor bets (low AS, high TE), and (4) closet indexers (low AS, low TE). Their finding that diversified and concentrated stock pickers outperformed while closet indexers underperformed—particularly after fees—challenged the conventional wisdom that tracking error was the dominant measure of active management quality.\n\nFrom a practical standpoint, active share has limitations. It is benchmark-sensitive: a fund's active share will differ materially depending on whether it is measured against the S&P 500, the Russell 1000, or a custom style benchmark. It also ignores sector tilts within the same securities, and it treats a 1% overweight in a $10 billion market cap stock identically to a 1% overweight in a $100 million stock. For large funds managing multi-billion dollar mandates, it becomes mechanically difficult to maintain high active share due to liquidity constraints and market impact considerations.", "example": "A US large-cap equity fund with $2 billion AUM holds 45 stocks. Its benchmark is the S&P 500 (500 stocks). For the 455 benchmark stocks not held in the portfolio, the portfolio weight is 0% vs. the benchmark weight averaging ~0.15% per name. For the 45 held stocks, the portfolio has average 2.2% weights vs. benchmark average of 0.4%. Calculating: overweights in 45 held stocks sum to approximately +82% and underweights (including zeros in the 455 not-held stocks) sum to approximately -82%. Active Share = 0.5 × 164% = 82%. This fund is legitimately active by the Cremers-Petajisto threshold. If the same manager held 450 of the 500 S&P stocks in near-benchmark proportions, active share might fall to 15-20%, indicating closet indexing despite charging active management fees.", "formula": "Active Share = 0.5 × Σ|w_portfolio(i) - w_benchmark(i)|", "formula_latex": null, "interactive_type": "calculator", "calculator_id": null, "related_terms": [ "adr-american-depositary-receipt", "cap", "equity", "liquidity", "market-impact", "preferred-stock", "price-to-book-ratio", "return-on-assets", "standard-deviation", "stock", "tracking-error" ], "backlinks": [ "adr-american-depositary-receipt", "days-to-cover", "market-capitalization", "portfolio-optimization", "stock-buyback" ], "cross_references": [ "cap", "equity", "liquidity", "market-impact", "standard-deviation", "stock", "tracking-error" ], "tags": [ "level:intermediate", "cat:equities" ], "asset_classes": [ "equities" ], "regulators": [], "see_also": [], "sources": [], "wordcount": 709, "checksum": "673d1e4586c94663", "version": "2026.05.03", "license": "CC-BY-4.0", "updated_at": "2026-09-07T02:15:24+00:00", "_links": { "self": "https://hedgefund.wiki/api/v1/terms/active-share", "jsonld": "https://hedgefund.wiki/api/v1/terms/active-share?format=jsonld", "markdown": "https://hedgefund.wiki/api/v1/terms/active-share?format=md", "graph": "https://hedgefund.wiki/api/v1/graph/active-share", "category": "https://hedgefund.wiki/api/v1/categories/equities", "schema": "https://hedgefund.wiki/schema/term.schema.json", "html": "https://hedgefund.wiki/#/terms/active-share" } }