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#2 I have no evidence that time spent learning and practicing a particular discipline influences skill level, but I am guessing that there is a correlation :). For majority of top schools aside from Barkley it is not a prerequisite to enter into CS program. From people I personally know which is not a representative sample I'd say 50/50 but it skewed by good number of them having Stats./Math as original major and then CS Masters or Ph.D.