I studied math & computer science at university (15 years ago) that was quite selective in CS admissions (probably filtering out ~80% of high-school graduates).

I studied math & computer science at university (15 years ago) that was quite selective in CS admissions (probably filtering out ~80% of high-school graduates). At least half of these already self-selected and 80% filtered students were not really able to program (and eventually flunked or transferred to other departments - although some people who could program flunked and some who couldn't graduated, I think the two groups were quite well correlated). I'm sure you can do better at teaching (and filtering) than the CS program at my university, but if such a focused group had a 50% failure rate I'm sure the general population is at the very least not better (remember that it's not just about 'raw' ability but also about interest and motivation).