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Portfolio Variance & Std Devの包括的な教育ガイドを準備中です。ステップバイステップの解説、数式、実例、専門家のヒントをお届けしますので、もうしばらくお待ちください。
Portfolio variance measures the total risk of a combination of investments by accounting for not just the individual riskiness of each asset, but also how they move in relation to each other. When two assets tend to move in the same direction at the same time, combining them does little to reduce risk. But when they move in opposite directions — or even just differently — mixing them creates a portfolio that is less volatile than any individual component. This is the mathematical engine behind diversification, and it is why holding a mix of stocks, bonds, real estate, and commodities can reduce overall portfolio risk without necessarily sacrificing return. This idea was formalised by Harry Markowitz in his 1952 paper 'Portfolio Selection,' which earned him the Nobel Prize in Economics in 1990. Before Markowitz, investors focused almost entirely on picking good individual securities. His insight was that the portfolio as a whole mattered more than any single stock — and that correlation between assets was the key to understanding combined risk. His framework, now called Modern Portfolio Theory (MPT), introduced the concept of the efficient frontier: the set of portfolios that offer the highest return for any given level of risk, or the lowest risk for any given return. Practically, portfolio variance is computed using each asset's weight, its individual variance (or standard deviation), and the pairwise covariances between every combination of assets. With two assets it is a simple formula; with twenty assets you need a 20×20 covariance matrix. Modern portfolio optimisation software handles this matrix algebra automatically, but understanding the underlying logic — that reducing average pairwise correlation reduces portfolio variance — is essential for any investor building a multi-asset strategy. Portfolio variance is also central to risk reporting in professional investment management. Factor models decompose portfolio variance into contributions from market risk, sector tilts, style exposures, and idiosyncratic risk, allowing portfolio managers to understand exactly where their risk is coming from and whether they are being compensated for it.
Two-asset: σ²_p = w₁²σ₁² + w₂²σ₂² + 2·w₁·w₂·ρ₁₂·σ₁·σ₂ N-asset: σ²_p = Σᵢ Σⱼ wᵢ·wⱼ·σᵢⱼ (matrix form: σ²_p = wᵀ·Σ·w) Portfolio Std Dev (σ_p) = √σ²_p Covariance: σᵢⱼ = ρᵢⱼ·σᵢ·σⱼ
- 1List every asset in the portfolio with its current market value, then compute each asset's weight as its value divided by total portfolio value.
- 2Gather the annualised standard deviation (volatility) for each asset — typically estimated from 3–5 years of monthly returns, then annualised (multiply monthly σ by √12).
- 3Construct the pairwise correlation matrix: for each pair of assets, calculate the correlation of their historical return series. Most portfolio software or spreadsheets can compute this from a return history.
- 4Convert correlations to covariances: σᵢⱼ = ρᵢⱼ × σᵢ × σⱼ for each pair.
- 5For two assets, apply the direct formula. For more assets, compute wᵀΣw using matrix multiplication — this is the inner product of the weight vector with the covariance matrix.
- 6Take the square root of portfolio variance to get portfolio standard deviation (σ_p) — the more intuitive risk measure expressed in percentage terms.
- 7Compare σ_p to the weighted average of individual σ values (Σwᵢσᵢ). The difference is the diversification benefit — how much risk you avoided by combining assets rather than holding each separately.
Weighted-average σ would be 0.60×18+0.40×6 = 13.2%; the portfolio achieves only 10.5% — saving 2.7% of risk through diversification.
The negative correlation between stocks and bonds (bonds often rally when stocks fall) is the source of the risk reduction. This is why the 60/40 portfolio became the default balanced allocation for pension funds and endowments for decades — it captures equity returns while meaningfully dampening volatility.
Two stocks in the same sector have very high correlation; diversification saves only ~1% of risk vs. the weighted average of 29%.
When assets are highly correlated, combining them produces minimal diversification benefit. Holding two tech stocks is barely different from holding one tech stock at twice the size. True diversification requires assets that respond differently to economic conditions — not just different company names in the same sector.
Adding gold (near-zero correlation with both) reduces portfolio volatility further than the 60/40 baseline.
The gold allocation brings its low correlation with both stocks and bonds into play. Even though gold itself is volatile (15%), because it moves independently of both equities and bonds, adding it reduces overall portfolio variance. This demonstrates how volatility of an individual asset matters less than its correlation to the rest of the portfolio.
In crises, cross-asset correlations surge toward 1.0, eliminating diversification precisely when you need it most.
This is the most important limitation of portfolio variance calculated from normal-period data: correlations are not stable. During the 2008 financial crisis and COVID March 2020 selloff, nearly every risk asset fell simultaneously as investors liquidated everything to raise cash. Models based on long-run average correlations dramatically underestimated risk during these episodes.
Constructing mean-variance efficient portfolios and computing the efficient frontier, representing an important application area for the Portfolio Variance in professional and analytical contexts where accurate portfolio variance calculations directly support informed decision-making, strategic planning, and performance optimization
Risk decomposition and attribution in institutional fund management, representing an important application area for the Portfolio Variance in professional and analytical contexts where accurate portfolio variance calculations directly support informed decision-making, strategic planning, and performance optimization
Regulatory capital modelling under Basel III for banks and Solvency II for insurers, representing an important application area for the Portfolio Variance in professional and analytical contexts where accurate portfolio variance calculations directly support informed decision-making, strategic planning, and performance optimization
Robo-advisor and target-date fund portfolio construction algorithms, representing an important application area for the Portfolio Variance in professional and analytical contexts where accurate portfolio variance calculations directly support informed decision-making, strategic planning, and performance optimization
Illiquid assets — infrequent pricing
{'title': 'Illiquid assets — infrequent pricing', 'body': "Assets priced infrequently (private equity, real estate) show artificially low measured correlation with public markets because their valuations lag. This 'smoothing effect' understates true variance and correlations. Models that use appraisal-based returns for private assets significantly underestimate true portfolio risk and overstate diversification benefits. Dimson (1979) correction or return-unsmoothing techniques help adjust for this."}
Fat tails and non-normal returns
{'title': 'Fat tails and non-normal returns', 'body': 'Standard portfolio variance assumes normally distributed returns. In reality, asset returns have fat tails — large moves occur far more often than a normal distribution predicts. During the 2008 crisis, the 2020 COVID crash, and the 2022 bond selloff, returns were 5–10 standard deviation events by normal distribution standards. Expected Shortfall (CVaR) and stress testing complement variance for capturing tail risk.'}
Estimation error in covariance matrices
{'title': 'Estimation error in covariance matrices', 'body': 'Covariance matrices estimated from historical data are noisy, especially for large portfolios. With 50 assets, you estimate 1,225 covariances — each carrying estimation error. Optimisers amplify these errors, producing unstable portfolios with extreme weights. Shrinkage estimators (Ledoit-Wolf), factor models, or robust optimisation methods reduce sensitivity to estimation error.'}
| Correlation (ρ) | Portfolio σ | Risk vs. Weighted Avg (17.5%) | Interpretation |
|---|---|---|---|
| −1.0 (perfect negative) | 2.5% | −15.0% | Maximum diversification — near-zero risk possible |
| −0.5 | 10.4% | −7.1% | Strong diversification benefit |
| 0.0 (uncorrelated) | 12.5% | −5.0% | Good diversification — independent assets |
| 0.5 | 14.4% | −3.1% | Moderate benefit — same-direction assets |
| 0.8 | 16.5% | −1.0% | Minimal benefit — highly correlated |
| +1.0 (perfect positive) | 17.5% | 0% | No diversification — identical to weighted average |
What is the difference between portfolio variance and portfolio standard deviation?
Portfolio variance (σ²_p) is the squared measure of risk — the unit is 'percent squared,' which is not intuitive. Portfolio standard deviation (σ_p = √σ²_p) converts this back to percentage terms that can be directly compared to asset returns. If your portfolio has an annual standard deviation of 12%, that means returns typically fall within ±12% of the mean in about two-thirds of years (one standard deviation). Both measure the same thing; standard deviation is more commonly used in practice because the units match those of returns.
Why does correlation matter more than individual volatility for diversification?
Adding a volatile asset to your portfolio can actually reduce overall portfolio risk if that asset has low or negative correlation with existing holdings. For example, gold has annual volatility of 15–20%, comparable to equities, yet adding 10–15% gold to a stock-bond portfolio typically reduces total portfolio volatility. The math makes this precise: the cross-product term (2·w₁·w₂·ρ·σ₁·σ₂) is subtracted when ρ is negative and added when ρ is positive. Correlation is literally the parameter that determines whether diversification works.
How many assets are needed for adequate diversification?
Research by Evans and Archer (1968) showed that most idiosyncratic (company-specific) risk is eliminated with about 20–30 randomly selected stocks. Beyond that, adding more stocks reduces risk marginally. However, a portfolio of 30 US stocks is only diversified against company-specific risk — it still has full exposure to US equity market risk, sector risk, and factor risks. True diversification requires crossing asset classes (stocks, bonds, real estate, commodities, alternatives) and geographies (domestic, international developed, emerging markets).
What is a covariance matrix and how do I build one?
A covariance matrix (Σ) is an N×N grid where each entry [i,j] shows the covariance between asset i and asset j. The diagonal entries are the individual variances (σᵢ²). The matrix is symmetric (entry [i,j] = [j,i]) because the covariance between A and B equals the covariance between B and A. To build one from historical data: collect monthly returns for all assets, compute pairwise covariances using Excel's COVAR function or Python's numpy.cov(), then annualise by multiplying by 12 (for monthly inputs).
What is the minimum variance portfolio?
The minimum variance portfolio is the combination of assets — specific to each asset's weights — that produces the lowest possible portfolio variance across all possible allocation combinations. It sits at the leftmost point of the efficient frontier. It is not necessarily the optimal portfolio (it sacrifices expected return), but it represents the maximum diversification benefit achievable given the assets available. For a two-asset case, the minimum variance weight for asset 1 is: w₁ = (σ₂² − σ₁₂) / (σ₁² + σ₂² − 2σ₁₂), where σ₁₂ is the covariance.
How does leverage affect portfolio variance?
Leverage amplifies portfolio variance by the square of the leverage ratio. If you borrow 50% of portfolio value to invest (1.5× leverage), portfolio variance increases by a factor of 1.5² = 2.25, and standard deviation increases by 1.5×. This means a 12% annualised standard deviation at 1× becomes 18% at 1.5×. Leverage cannot be diversified away — it scales all components proportionally. This is why highly leveraged funds (e.g., 2× and 3× ETFs) have dramatically higher volatility than their underlying indices.
What is the difference between portfolio variance in MPT and factor model risk?
MPT computes total portfolio variance from raw return history. Factor models (CAPM, Fama-French, Barra) decompose portfolio variance into systematic components (market risk, size risk, value risk, momentum risk) and idiosyncratic component. Factor model variance gives more insight into why you have the risk you have — is it market beta, sector concentration, or factor tilts? For portfolio management, factor risk decomposition is more actionable; for precise variance computation, the covariance matrix approach is more accurate.
プロのヒント
Run two portfolio variance calculations: one using long-run average correlations (normal period) and one using crisis-period correlations (assume all risky asset correlations spike to 0.80+). The gap between these two numbers is your 'diversification credit' — the amount of risk reduction that may disappear in a real crisis. Size your risk budget conservatively against the crisis-correlation scenario.
ご存知でしたか？
Harry Markowitz reportedly developed his portfolio variance framework as a PhD student in the 1950s while reading John Burr Williams' 'Theory of Investment Value' in a university library. His insight — that investors should care about portfolios, not individual stocks — came in one afternoon. His Nobel Prize winner's speech noted that Milton Friedman initially suggested the work was not economics. Decades later, Markowitz's covariance matrix is embedded in every institutional investment process in the world.