Coding classes in Ceredigion

The learner downloads the Aberporth file, reads each month's count of air frost days, and skips months marked missing or provisional. Then the program asks, for each calendar month, in how many of the 69 years there was at least one frost. Winter looks as expected: January had frost in 59 of the years, December in 52. The spring and autumn edges are more interesting. April had frost in 21 years, most recently in 2013. October managed it only once, in 1983. And from May to September the answer is zero, every time.

Zero in 69 does not mean the chance is zero. The rule of three says that when something has not happened in n independent tries, you can be about 95 per cent confident its chance per try is below 3 divided by n. Here that is 3 over 69, about 4.3 per cent, so a frosty June at Aberporth is at most roughly a one-in-23-year event, and possibly far rarer. The learner checks the rule against the exact calculation, the Clopper-Pearson bound, which gives 4.2 per cent: the shortcut is remarkably good. With only 10 years of zeros the bound would be much looser, 30 per cent by the rule and 26 by the exact method.

October is the cautionary tale. Anyone reading the record at the end of 1982 would have seen 26 Octobers without frost and might have called it impossible; the next year proved otherwise. The rule of three would have said at most about 12 per cent a year, which was honest. The learner also notes the assumption that years are independent and conditions unchanged, which a warming climate may not respect.

Turning "never" into an honest number is a strong reason for Ceredigion teenagers to keep learning to code in 2026. The longer argument is in why teenagers should still learn to code in 2026.