Interchangeable ch/sh and k/t characters

[MarcoP]

"t" seems to occur significantly less frequently with the “aiin” family than "k".

This is true of almost anything, when numbers are high enough, because there are less t than k.

I agree. It's better to work with percent figures, since character counts differ.

But there’s another piece of “evidence” that, for example, t and k aren’t interchangeable:
k before aiin (20.6 percent) (n=810)
t before aiin   (9.0 percent) (n=356)
k before ain  (38.1 percent) (n=645)
t before ain  (12.4 percent) (n=210)
"t" seems to occur significantly less frequently with the “aiin” family than "k". "t" is more likely to appear with ‘o’ and "ch."

The most informative part of JoJo's post was the last line: to/ko tch/kch have similar counts, though t is only ~60% as frequent as k.

Anyway, I am not sure this means that the two "aren’t interchangeable" - they have different preferences in some contexts, but they still look very much interchangeable to me.