What factor(s) enter the decisions about the composition of a portfolio of marketable securities?

The important factor is the maturity of each security in the portfolio. Marketable securities should be selected with an eye toward when the money will be needed again in order to insulate the company from market price fluctuations. In this way, the company will receive a known face value when each security matures. Since the values of all marketable securities in an economy are closely tied to interest rates, it is not possible to use statistical portfolio techniques (betas) to reduce the risk of this kind of portfolio.