Markowitz's graphical representation was, however, only a way of explaining the underlying idea. To calculate the "efficient frontier" in any particular case (given a set of securities and estimates of their expected returns, variances, and correlations) is computationally demanding. When the set of securities is even moderately large, it is beyond the capacity of an unaided human being. Because the problem fell into a mathematical domain that operations researchers were only beginning to explore, there was no "off-the-shelf" algorithm Markowitz could turn to: it was difficult, innovative work.

Donald MacKenzie, An Engine, Not a Camera, loc. 216