CFA Level 1 (2009) - 1

Study Session 2

Cross-Reference to CFA Institute Assigned Reading #8 - Probability Concepts

Example: Calculating variance from a probability model

Calculate the variance and standard deviation of EPS for Ron's Stores using the probability distribution ofEPSfromthe table in the previous example.

Answer:

Variance of EPS for Ron's Stores is:

(1.00- 1.28)2 =0.0736

The standard deviation of EPS for Ron's Stores is:

crEPS = (0.0736)112 =0.27

Note that the units of standard deviation are the same as that of EPS, so we would say that the standard deviation of EPS for Ron's Stores is £0.27.

LOS 8.h: Explain the use of conditional expect;;tion in investmcnt applications.

Conditional expected values are calculated using conditional probabilities. In investments, forecasts are frequently made using expected values for a stock's return, earnings, and dividends. After the initial forecast, new and relevant information may surface that can affect the forecasted value(s). When this happens, the original forecast must be refined, and it is done using conditional expected values. As the name implies, conditional expected values are expected values that are contingent upon the occurrence of some other evenr.

LOS 8.i: Diagram an investment problem, using a tree diagram.

You might well wonder where the recurns and probabilities used in calculating expected values come from. A general framework called a tree diagram is used co show the probabilities of various outcomes. In Figure 3, we have shown estimates of EPS for four different outcomes: a good economy and relatively good results at the company, a good economy and relatively poor results at the company, a poor economy and relatively good results at the company, a poor economy and relatively poor results at the company. Using the rules of probability we can calculate the probabilities of each of the four EPS ou tcomes shown in the boxes on the righ t- hand side of the "tree."

Srudy Session 2

Cross-Reference to CFA Institute AssignedReading #8 - Probability Concepts

Figure 3: A Tree Diagram

EPS =$1.80

Prob = 18%

EPS =$1.70

Prob =42%

Expected

EPS =$1.51

EPS =$1.30

Prob =24%

poor economy = 40%

EPS = $1.00

Prob = 16%

The expected EPS of $1. 51 is simply calculated as:

0.18 x 1.80 + 0.42 x 1.70 + 0.24 x 1.30 + 0.16 x 1.00 = $1.51

Note that the probabilities of the four possible outcomes sum to 1.

COVARIANCE AND CORRELATION

The variance and standard deviation measure the dispersion, or volatility, of only one variable. In many finance situations, however, we are interested in how two random variables move in relation to each other. For investment applications, one of the most frequently analyzed pairs of random variables is the returns of two assets. Investors and managers frequently ask questions such as, "what is the relationship between the return for Stock A and Stock B?" or "what is the relationship between the performance of the S&P 500 and that of the automotive industry?" As you will soon see, the covariance and correlation are measures that provide useful information about how two random variables, such as asset returns, are related.

LOS 8.j: Calculate and interpret covariance and correlation.

Covariance is a measure of how two assets move together. It is the expected value of the product of the deviations of the two random variables from their respective expected values. A common symbol for the covariance between random variables X and Y is Cov(X,Y). Since we will be mostly concerned with the covariance of asset returns, the following formula has been written in terms of the covariance of the return of asset

and the return of asset j, Rj :

Cov(R,R) = E{[R - E(R)] [R. - E(R)]}

Study Session 2

Cross-Reference to CFA Institute Assigned Reading #8 - Probability Concepts

The following are properties ofthe covariance:

•The covariance is a general representation of the same concept as the variance. That is, the variance measures how a random variable moves with itself, and the covariance measures how one random variable moves with another random variable.

Var{RA)·

The covariance may range from negative infinity to positive infinity.

To aid in the interpretation of covariance, consider the returns of a stock and of a put option on the stock. These two returns will have a negative covariance because they move in opposite directions. The returns of two automotive stocks would likely have a positive covariance, and the returns of a srock and a riskless asset would have a zero

covariance because the riskless asset's returns never move, regardless of movements in the srock's return. While the formula for covariance given above is correct, the method of computing the covariance of returns from a joint probability model uses a probabilityweighted average of the products of the random variable's deviations from their means for each possible outcome. The following example illustrates this calculation.

Example: Covariance

Assume that the economy can be in three possible states (S) next year: boom, normal, or slow economic growth. An expert source has calculated that P{boom) = 0.30, P(normal) = 0.50, and P(slow) = 0.20. The returns for Stock A, RA' and Stock B, RB, under each of the economic states are provided in the probability model below. What is the covariance of the returns for Stock A and Stock B?

Probability Distribution of Returns

Answer:

First, the expected returns for each of the stocks must be determined.

E(RA ) = (0.3){0.20) + (0.5){0.12) + (0.2){0.05) = 0.13

E(RB) = (0.3)(0.30) + (0.5)(0.10) + (0.2)(0.00) = 0.14

The covariance can now be computed using the procedure described in the following table.

Study Session 2

Cross-Reference to CFA Institute Assigned Reading #8 - Probability Concepts

Covariance Computation

.Cov{RAIR~)=~P(S) x [RA - E(RA)] x TRB':'E(ItIl)r:=0,00~8b

The preceding example illustrates th~use'ofajoint probability function;,Aj~int.·:: probabilityfunct.ion for tWo random variables gives the probability ofthejoint " . occurrence of specified outcomes. In this case, we only had three jointprobabilities:

P(RA = 0.2 and RB = 0.3) = 0,30

P(RA = 0.12 and RB = 0.1) = 0.50

P(RA = 0.05 and RB = 0.0) = 0.20

Joint probabilities are often presented in a table such as the one shown in the following figure. According to the following figure, P(RA = 0.12 and RB = 0.10) =' 0.50. This is the probability represented in the cell at the imersectionof the column labeled RB = 0.10 and the row labeled RA =0.12. Similarly, P(RA = 0.20 and R B =

In more complex applications, there would likely be positive values where the zeros appear in the previous table. In any case, the sum of all the probabilities in the cells on the table must equal 1.

In practice, the covariance is difficult to interpret. This is mostly because it can take on extremely large values, ranging from negative to positive infinity, and, like the variance, these values are expressed in terms of square units.

To make the covariance of twO random variables easier to interpret, it may be divided by the product of the random variable's standard deviations. The resulting value is called the correlation coefficient, or simply, correlation. The relationship between covariances,

Study Session 2

Cross-Reference to CFA Institute Assigned Reading #8 - Probability Concepts

standard deviations, and correlations can be seen in the following expression for the correlation of the returns for asset i and j:

The correlation between two random return variables may also be expressed as p(Ri,Rj), or p.I.)'.

Properties ofcorrelation of two random variables Rj and Rj are summarized here:

•Correlation measures the strength of the linear relationship between twO random variables.

•Correlation has no units.

• The correlation ranges from -1 to + 1.

•That is, -1 ~ Corr(Ri, Rj)~ +1

•If Corr(Ri' Rj) = 1.0, the random variables have perren positive correlation. This means that a movement in one random variable results in a proportional positive

movement in the other relative to its mean.

•If Corr(Ri, R) = -1.0, the random variables have perfect negative correlation. This means that a movement in one random variable results in an exact opposite

proportional movement in the other relative to its mean.

•If Corr(Ri, Rj) = 0, there is no linear relationship between the variables, indicating

Using our previous example, compute and interpret the correlation of the returns for

First, it is necessary to convert the variances to standard deviations.

a(RA ) = (0.0028)'12 = 0.0529

a(RB) = (0.0124)'12 =0.1114

Now, the correlation between the returns of Stock A and Stock B can be computed as follows:

0.0058

The fact that this value is close to +1 indicates that the linear relationship is not only positive but very strong.

Srudy Session 2

Cross-Reference to CPA Institute Assigned Reading #8 - Probability Concepts

LOS 8.k: Calculate and interpret the expected value, variance, and standard deviation of a random variable and of returns on a portfolio.

The expected value and variance for a portfolio of assets can be determined using the properties of the individual assets in the portfolio. To do this, it is necessary to establish the portfolio weight for each asset. As indicated in the formula below, the weight, w, of portfolio asset i is simply the market value currently invested in the asset divided by the current market value of the entire portfolio.

market value of investment in asset i

w· =

I

market value of the portfolio

Portfolio expected value. The expected value of a portfolio composed of n assets with weights, wi' and expected values, Ri• can be determined using the following formula:

N

;=1

More often, we have expected returns (rather than expected prices). When the Rj are returns, the expected rerum for a portfolio, E(Rp), is calculated using the asset weights and the same formula as above.

Portfolio variance. The variance of the portfolio return uses the porrfoJ in weights also. bur in a more complicated way:

N N

Var(R p ) = LLWjWjCov(R j ,R j )

i=1 ;=!

The way this formula works, particularly in its use of the double summation operator, LL, is best explained using 2-asset and 3-asset portfolio examples.

Exainple: Yarianceof a 2-asset portfolio

Symbolically express the variance of a portfolio composed of risky asset A and risky assetB.

Answer:

Application of the variance formula provides the following:

Var(R) = wAwACov(RA>RA) + wAwBCov(RA>RB) + wBwACov(RB,RA) +

w wBCov(RB,R ) B B

Now, since Cov(RA>RB)=Cov(RB,RA), and Cov(RA>RA ) = a 2 (RA), this expression reduces to the following:

Var(~p)=w:la2(RA) + WB2a2(RB) + 2w w Cov(RA>R )

A B B

SinceCov(RA>RB)= a(RB)a(RA)p(RA>RB), anotherway to present this formula is:

Srudy Session 2

_Cross-Reference to CFA Institute Assigned Reading #8 - Probability Concepts

Example: Variance of a 3-asset ,portfolio

. A portfolio composed of risky assets A, B, andC will have a variance of return determined as:

= wAWACov(RA'R.A) + WAwBCov(RA,'R ) + W weCov(RA'Rc)'" . B A

+wBw,ACov(RB,RA) +wBwBCov(RB,RB) + wBweCov(RB,Rc)

+weWACov(Rc,R.A) +wewBCov(Rc,RB) + weweCov(Rc,Rc)

which can be reduced to thefollowing:expression:

A portfolio composed of four assets will have four w j2a 2(R) terms and six

2wjw jCov(Ri'R) terms. A portfolio with five assets will have five wj2a 2(R) terms and ten 2wjw/~ov(Ri,Rj)terms. In fact, the expression for the variance of an n-asset

portfolio will have n(n - 1)/2 unique Cov(Ri'Rj) terms since Cov(Rj,Rj) = Cov(Rj,Rj ).

Professor's Note: I wouLd expect that ifthere is a probLem on the exam that requires the caLcuLation ofthe variance (standard deviation) ofa portfoLio of risky assets. it wouLd invoLve onLy two risky assets.

The foHowing formula is useful when we want to compute covariances, given correlations and variances.

LOS 8.1: Calculate and interpret covariance given a joint probability function.

Example: Expected value, variance, and covariance

What is the expected value, variance, and covariance(s) for a portfolio that consists of $400 in Asset A and $600 in Asset B? The joint probabilities of the returns of the two assets are in the following figure.

Probability Table

Study Session 2

Cross-Reference to CFA Institute Assigned Reading #8 - Probability Concepts

Answer:

The asset weights are:

wA = $400/ ($400 +$600) = 0.40

WB = $600/($400 +$600) =0.6.0

The expected ~et~rns for the individual assets are determined as:

E(RA) = (0.15)(0.20) + (0.60)(0.15) + (0.25)(0.04) = 0.13

E(RB) = (0.15)(0.40) + (0.60)(0.20) + (0.25)(0.00) = 0.18

The variances for the individual asset returns are determined as:

Cov(RA , RB) 0.15(0.20 - 0.13)(0.40 - 0.18)

+0.60(0.15 - 0.13)(0.20 - 0.18)

+0.25(0.04 - 0.13) (0.00 - 0.18) 0.0066

Study Session 2

Cross-Reference to CFA Institute Assigned Reading #8 - Probability Concepts

Using the weights w A = 0.40 and wB=0.60, the expected return,and variance of the portfolio are computed as:

Please notethataSc tedious as this example was, ifmore of the cells in thejoint probability matrix were not zero, it could have been even more tedious.

Example: Correlation and covariance

Consider a portfolio of three assets, X, Y, and Z, where the individual market value of these assets is $600, $900, and $1,500, respectively. The market weight, expected return, and variance for the individual assets are presented below. The correlation matrix for the asset returns are shown in the following figure. Using this information, compute the variance of the portfolio return.

Answer:

The expected return for the portfolio may be determined as:

E(Rp )= (0.20)(0.10) + (0.30)(0.12) + (0.50)(0.16)

E(Rp)= 0.136

Study Session 2

Cross-Reference to CFA Institute Assigned Reading #8 - Probability Concepts

The variance of a 3-asset portfolio return is determined using the formula:

wia2(Rx) + w/a2 (Ry) + wla2(Rz }+ 2wXwyCov(Rx,Ry) + 2wxwzCoY(Rx,Rz ) + 2wywZCov(Ry>Rz )

Herewe must make use of the relationship Cov(Ri'R) = a(Ri)a(Rj)p(Rj>Rj), since we are not provided with the covariances. J

Let's solve for the covariances, then substitute the resulting values into the portfolio return variance equation.

Cov(Rx,Ry) = (0.0016)!h(0.0036)!h(0.46) =0.001104

Cov(Rx,Rz) = (0.0016)!h(0.0100)!h(0.22) = 0.000880

Cov(Ry>Rz) = (0.0036)1,2(0.0100)1,2(0.64) = 0.003840

Now we can solve for the variance of the portfolio returns as:

Var(Rp) = (0.20)2(0.0016) + (0.30)2(0.0036) T (0.50)2(0.01) + (2)(0.2)(0.3)(0.001104) + (2)(0.2)(0.5)(0.00088) + (2)(0.3 )(0.5)(0.00384)

Var(Rp) = 0.004348

The standard deviation of portfolio returns = (0.004348) 1/2 = 0.0659 = 6.59%

Example: Covariance matrix

Assume you have a portfolio that consists of Stock S and a put option, 0, on Stock S. The corresponding weights of these portfolio assets are W s = 0.90 and W o =0.10.

Using the covariance matrix provided in the following figure, calculate the variance of the return for the portfolio.

Returns Covariance for Stock S and Put 0

Covariance Matrix

Returns

0.0011 -0.0036

-0.0036 0.016