A $500B 13F filing with 3,000 positions. Is this a sophisticated stock picker with thousands of research-driven bets? Or an index fund that mechanically holds every stock in the S&P 500?
The 13F format doesn't tell you. But three quick tests will.
Test 1: Top-holding overlap with S&P 500
Pull the filing's top 10 holdings. Compare them to the S&P 500's top 10 by market cap.
| Overlap | Interpretation |
|---|---|
| 9-10 of 10 match | Almost certainly passive or benchmark-hugging |
| 6-8 of 10 match | Hybrid — passive core with active tilts |
| 3-5 of 10 match | Active — deliberate deviation from the index |
| 0-2 of 10 match | Highly active — this is a real stock picker |
Example: If the top 10 is AAPL, MSFT, NVDA, AMZN, GOOGL, META, BRK, AVGO, LLY, JPM — that's the S&P 500. No one picked those stocks. The index did.
Counterexample: If the top 10 is TSLA, SHOP, ROKU, COIN, PLTR, AMD, HOOD, SQ, RBLX, DKNG — that's ARK Invest. Every name is a deliberate, researched bet.
Test 2: Concentration ratio
Calculate the top-5 and top-10 concentration:
| Top-10 concentration | Typical filer type |
|---|---|
| 15-25% | Passive index fund (matches market concentration) |
| 25-35% | Large diversified active manager |
| 35-50% | High-conviction active manager |
| 50%+ | Concentrated fund or market maker |
Why this works: The S&P 500's top-10 concentration is ~35%. A filing with similar concentration AND similar names is passive. A filing with 50% concentration in different names is active and concentrated.
Caveat: Market makers (Jane Street, Susquehanna, Optiver) can show extreme concentration but aren't stock pickers — they're holding inventory. Check the institution name.
Test 3: Position count relative to AUM
The ratio of positions to AUM reveals the portfolio construction approach:
| AUM per position | Interpretation |
|---|---|
| <$10M average | Extremely broad — likely index or quantitative |
| $10-100M average | Broad active or multi-strategy |
| $100M-1B average | Concentrated active |
| >$1B average | Highly concentrated or mega-cap only |
Example: Vanguard at $6.9T with 4,000+ positions = ~$1.7B average. But this is misleading — most of the AUM is in the top 50 names. Better to use this test for mid-size filers.
Better example: Dodge & Cox at $185B with 222 positions = ~$835M average. That's a concentrated active manager where every position is a researched bet.
Combining the three tests
| Filer | Top-10 overlap | Concentration | Positions/AUM | Verdict |
|---|---|---|---|---|
| Vanguard | 10/10 | ~index | Very broad | Passive |
| Fidelity | 8/10 | Above index | Broad | Hybrid |
| Jennison | 6/10 | 47.6% top-10 | Moderate | Active growth |
| Dodge & Cox | 2/10 | Moderate | 222 positions | Active value |
| ARK Invest | 0/10 | Very high | Concentrated | Thematic active |
| Jane Street | 4/10 (ETF heavy) | ~40% | Very broad | Market maker |
Why this matters
The interpretation of every signal in a 13F depends on whether the filer is passive or active:
- New position in a passive filing = index addition (no signal)
- New position in an active filing = fresh conviction (signal)
- Position increase in a passive filing = stock price went up (no signal)
- Position increase in an active filing = manager is adding (signal)
- High concentration in a passive filing = market is concentrated (macro fact)
- High concentration in an active filing = manager has conviction (stock signal)
Without classifying the filer first, you'll misread every data point that follows.
Originally published at 13F Insight
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