Читать книгу Asset Allocation - William Kinlaw, Mark P. Kritzman - Страница 60

The final step is to select the portfolio that best suits our aversion to risk, which we call the optimal portfolio. As we discussed earlier in this chapter, the theoretical approach to identifying the optimal portfolio is to specify how many units of expected return we are willing to give up to reduce our portfolio's risk by one unit. We would then draw a line with a slope equal to our risk aversion and find the point of tangency of this line with the efficient frontier. The portfolio located at this point of tangency is theoretically optimal because its risk/return trade-off matches our preference for balancing risk and return.

In practice, however, we do not know intuitively how many units of expected return we are willing to sacrifice in order to lower variance by one unit. Therefore, we need to translate combinations of expected return and risk into metrics that are more intuitive. Because continuous returns are approximately normally distributed,⁹ we can easily estimate the probability that a portfolio with a particular expected return and standard deviation will experience a specific loss over a particular horizon. Alternatively, we can estimate the largest loss a portfolio might experience given a particular level of confidence. We call this measure value at risk. We can also rely on the assumption that continuous returns are normally distributed to estimate the likelihood that a portfolio will grow to a chosen value at some future date. Table 2.5 shows the likelihood of loss over a five-year investment horizon for the three efficient portfolios, as well as value at risk measured at a 1% significance level, which means we are 99% confident that the portfolio value will not fall by more than this amount.

TABLE 2.5 Exposure to Loss

Risk statistics for end of five years
	Conservative (%)	Moderate (%)	Aggressive (%)
Probability of Loss (return below 0%)	2.5	6.7	10.8
1% Value at Risk	5.1	17.0	28.7

If exposure to loss were our only consideration, we would choose the conservative portfolio, but by doing so we forgo upside opportunity. One way to assess the upside potential of these portfolios is to estimate the distribution of future wealth associated with investment in each of them.

Table 2.6 shows the probable terminal wealth as a multiple of initial wealth at varying confidence levels 15 years from now. For example, there is only a 5% chance that our nominal wealth would grow to less than 1.4 times initial wealth in 15 years, assuming we invest in the moderate portfolio. And there is a 5% chance (1 – 0.95) that our wealth could grow to a value as high as 5.2 times initial wealth.

TABLE 2.6 Distribution of Wealth 15 Years Forward (as a Multiple of Initial Investment)

Terminal wealth, multiple, end of 15 years
Confidence Level (%)	Conservative	Moderate	Aggressive
1	1.3	1.1	0.9
5	1.5	1.4	1.3
10	1.7	1.7	1.6
25	2.0	2.1	2.2
50	2.3	2.7	3.2
75	2.7	3.6	4.5
90	3.2	4.5	6.3
95	3.5	5.2	7.6
99	4.1	6.8	11.0

These estimates of future wealth ignore any contributions or disbursements that may be added to or subtracted from the portfolios. To estimate future wealth taking cash flows into account, we would need to simulate the portfolios' performance between all cash flows throughout our investment horizon.

By mapping the portfolios' expected returns and standard deviations onto estimates of exposure to loss and the distribution of future wealth, we should have a clear idea of the merits and limitations of each portfolio. It is important to keep in mind, though, that there is no universally optimal portfolio; it is specific to each investor. If our focus is to avoid losses, the conservative portfolio might be optimal. If, instead, we believe that we can endure significant losses along the way in exchange for greater opportunity to grow wealth, then we might choose the aggressive portfolio. If our goal is to limit exposure to loss, yet still maintain a reasonable opportunity to grow wealth, then perhaps the moderate portfolio would suit us best.

Asset allocation is a complex process, yet one we should not ignore. Our intent in this chapter is to present the theoretical foundation of asset allocation as well as to discuss the practical implementation of this theory at its most basic level. In subsequent chapters, we describe various refinements to the basic approach described here. But before we move on to these refinements, we discuss some fallacies about asset allocation and do our best to dispel them.