Apples-to-apples comparison

 Apples-to-apples comparison would be used to describe a comparison between two things that are similar in nature, for example: comparing the acceleration of one mid-sized car to that of another mid-sized car. An apples-to-oranges comparison would be a comparison between two things that are not similar: comparing the acceleration of a mid-sized car to that of a bus.

ref: http://forum.wordreference.com/showthread.php?t=534368