No stock analyst could plausibly estimate half a million correlations. Statistical analysis of past correlations was not enough, even if the data for it had been easily available, which in the 1950s they were not, because what was needed for portfolio selection were future correlations. Even if estimates of the latter could somehow be produced, the limited sizes of computer memories in the 1950s and the early 1960s put the resultant computation well beyond the bounds of the feasible.

Donald MacKenzie, An Engine, Not a Camera, loc. 230