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"Which" vs "what" - what's the difference and when should you use one or the other?

Q: What's the rule-of-thumb on "for which" vs. "for what" usage? I recently wrote the sentence:

"For which data is it appropriate to use method A instead of method B"

This seems right. But I can't articulate why "for what" seems wrong.

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marked as duplicate by aedia λ, kiamlaluno, JSBձոգչ, Dusty, waiwai933 Sep 1 '11 at 16:36

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Generally, use "which" to select from a confined set of possibilities where all are mentioned. Use "what" to select from an open set or where the possibilities have not been mentioned.

There is some overlap, such as where there's an implied confined set. For example "For which day of the week are our reservations?" Here the set is confined but not mentioned. "What" works about as well.

Sometimes this difference changes the meaning. If there are three sandwiches on the table and I say "What do you want for lunch?" you are free to select something other than the sandwiches. If I say "Which do you want for lunch?" you have to choose from the offered set.

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