The average self-directed investor holds 15–20 positions. Research shows that portfolios under 30 positions in correlated sectors carry concentration risk equivalent to holding 5–6 uncorrelated positions. That gap between perceived and actual diversification is where portfolios quietly deteriorate.
Position Count Is Not Diversification
Holding 20 positions across three correlated sectors provides less risk reduction than 8 positions across uncorrelated asset classes. The mathematics are unambiguous: when correlations rise toward 1.0 during drawdowns, concentrated portfolios behave as though they hold a fraction of their stated positions.
A 2024 study by Vanguard found that 73% of self-directed portfolios had effective diversification ratios below 0.4 — meaning their real diversification was less than half of what position count suggested.
How to Measure Concentration Risk Accurately
Herfindahl-Hirschman Index (HHI): Sum the squares of each position's weight as a decimal. An HHI above 0.15 indicates meaningful concentration. Above 0.25, single-event risk dominates.
Top-3 weight test: Calculate the combined weight of your three largest holdings. If it exceeds 40%, a single earnings miss or regulatory action could trigger outsized losses. Above 60%, the portfolio behaves like a leveraged bet on a handful of names.
Correlation-adjusted sector exposure: Raw sector weights understate concentration when holdings within a sector are highly correlated. Correlation-adjusted measures of concentration can detect exposure that simple sector breakdowns miss.
What Published Research Says About Single-Stock Risk
JP Morgan Asset Management's Agony & Ecstasy analysis of Russell 3000 constituents (1980–2020) reports that approximately 40% of stocks experienced a permanent decline of 70% or more from their peak with no subsequent recovery — a base-rate observation about single-stock risk.
What the Health Score Penalises
KlirInvest's Health Score (model portfolio_health_score@2.1.0) combines six weighted convex penalties:
- Volatility — annualised portfolio standard deviation
- Tail risk (CVaR) — Expected Shortfall in the worst 5% of scenarios
- Concentration — largest single-position weight and top-5 weight sum
- Correlation stress — instability of pairwise correlations under stress
- Drawdown — depth of, and proximity to, the maximum drawdown
- Data confidence — quality of inputs behind this score: price-history depth, outliers, and coverage (a data-quality proxy, not market liquidity)
The six penalties are combined as "Health Score = 100 − Σ(weight × penalty)". A score below 50 indicates material structural weakness across one or more of those dimensions.
Other diversification concepts — for example HHI, sector balance, geographic distribution, and asset-class balance — are useful when interpreting your portfolio but are not direct inputs to this score.
What Educational Frameworks Suggest About Concentration
Common educational frameworks describe practices such as: auditing dollar-weighted exposure on a regular cadence (not just position count); reviewing whether single positions exceed self-imposed limits (institutional risk frameworks often reference 5–8% caps as a reference point, not a recommendation); examining whether diversification reflects independent return drivers rather than ticker count; and modelling sector-specific shocks against the portfolio. These are descriptions of what frameworks discuss — not instructions for any individual portfolio.
Concentration risk does not announce itself. It accumulates as winners grow and losers shrink. The time to measure it is before the next drawdown, not after.
See how your portfolio scores — run your free analysis at [KlirInvest](/free-tools?tab=analysis).