What's the difference between "case by case" and "case to case"?
I often hear the former from my Japanese students. When I asked them where they got the phrase, they always say they learned it from their high school grammar lessons.
This is probably due to difficulty with prepositions, for which it is harder to learn by rules than by experience! The preposition "by" is used in "X by X" where "X" is a noun to adverbially specify that the main verb is performed to each "X" in the context one at a time. Some examples are:
one by one (one at a time)
line by line (one line at a time)
case by case (one case at a time)
day by day (one day at a time)
In contrast, the adverbial phrase "X to X" specifies that the main verb is performed to the "X"s in the context from one to another. It need not imply that all "X"s are involved, but simply conveys the process of going from one to another, usually repeatedly.
[from] house to house (repeatedly out from a house into another)
[from] time to time (occasionally)
[from] cover to cover (start of a book to the end)
In the Indian sub-continent,'on a case to case basis'is often used,especially, in offices.I don't think it sounds awkward and I don't think it's non-standard!There was a time when 'weightage'was frowned upon but now it's increasingly being used,particularly,in the Sub-continent.And,for Pete'sake,who makes the rules!A rule may vary from place to place e.g.'gotten'has no applicability in British English!