An idiom for "beginning in the wrong end"

Say that in order to determine what B is, you analyze A. But then someone analyzes B in order to determine what A is. In my language, we would say that this procedure "begins in the wrong end". What is an appropriate English idiom or phrase here? The only one I can think of now is to say that the procedure has been "turned on its head".

EDIT: I intend to use the appropriate phrase or idiom in a scholarly paper, so it needs to be polite enough for that context.

The simplest and most direct translation might be "to begin at the wrong end" or "to start at the wrong end". There's no need to get too idiomatic, if you're writing formally, and I believe those phrases would strike a native English speaker as correct in the context you've specified.

There is an idiom that conveys the idea: to put the cart before the horse

• Fig. to have things in the wrong order; to have things confused and mixed up. (Also with have.)

• to do things in the wrong order

^{Source: http://idioms.thefreedictionary.com}

Wikipedia says:

The idiom is used in a context which reverses the usual chronological order of A and B.