Risiko kann man berechnen und reduzieren. Das Risiko, das sich mit Investitionen in Aktien, Bonds oder Schuldverschreibungen verbindet, kann gewichtet und balanciert werden: Diversifikation, Streuung und Ko-Varianz sind die wichtigsten Parameter, auf denen die Portfolio-Theorie basiert, jene Theorie, die es Investoren erlaubt, durch eine Kombination von Assets, die unterschiedliche Risiken aufweisen, den eigenen Profit zu maximieren:
“The fundamental insight of portfolio theory is that in general the process of diversification enables investors to improve the risk-return trade-off. Crucial to this process is consideration of both the number of securities and the degree of correlation amongst the individual securities that make up a portfolio. A well-diversified portfolio will tend to be one that is made up of securities that have a tendency to move in opposite directions or have low correlations, rather than securities that tend to move in similar directions and with high correlations. To the extent that shares are likely to move in similar directions, then shares that have low positive correlations provide better scope for diversification than shares that have a higher positive correlation.
We have seen that the correlation lies between -1 and +1. In the extreme case when securities are perfectly correlated (correlation coefficient +1), then diversification will not result in any risk reduction; whilst the other extreme of perfect negative correlation (correlation coefficient -1) it is possible by choice of appropriate weightings between the two securities to totally eliminate portfolio risk. While naive diversification can lead to substantial risk reduction to a portfolio, the process of efficient diversification by taking account of correlations and standard deviations between securities can enable investors to obtain an even better risk-return trade-off” (Pilbeam, Finance and Financial Markets, p.183).