reexamine the conditions for marking the ratio insignificant

Currently it is done like this:

https://github.com/andrewrk/poop/blob/b01058a8081e5584f1c28f74c59643e413b562df/src/main.zig#L365

but maybe there is better way to do it, that is more well accepted in the field of statistics.

(perhaps std deviation should be involved?)