Chapter: 12 Portfolio Theory & The Benefits of Diversification

Section: 2 Mean-Variance Analysis and Diversification

In mean-variance analysis, we use the expected returns, variances, and co-variances of individual investment opportunity returns to analyzing the risk-return trade-off of combinations, or portfolios, of these individual investments. The assumptions of mean-variance analysis are:

  • All investors are risk averse

  • Asset expected returns, asset return variances, and covariance can be calculated.

  • Investors can create optimal portfolios by only knowing expected returns, variances, and covariances.

  • No frictions or trading costs exist.

I. Expected Return: The expected return on a portfolio of two assets is simply the weighted average of the returns on the individual assets, weighted by their portfolio weights:

E(Rp)  = W1E(R1) + W2E(R2)

Where:
E(Rp)  = Expected return on portfolio
E(R1)  = Expected return on asset 1
E(R2)  = Expected return on asset 2
W1  = Percentage of total portfolio value invested in asset 1
W2  = Percentage of total portfolio value invested in asset 2

II. Standard Deviation: The standard deviation of a portfolio of two assets is not a simple weighted average of the asset standard deviation; it is also a function of the correlation between the returns of the two assets:

σp  =  [W12 σ12 + W22 σ22 + 2W1W2p 12 σ1 σ2]

Where:
σ1  =  Standard Deviation of the expected return on asset 1
σ2  =  Standard Deviation of the expected return on asset 2
ρ12  =  Correlation between the expected returns on assets 1&2

III. Correlation: Correlation can be defined as the percentage of variability in one variable (dependent variable) that is explained by another variable (independent variable). It is calculated as:

Corr (Ri, Rj)  =  Covariance (Ri, Rj)

σ(Ri) σ(Rj)