AHP and ANP
Thomas Saaty compares apples by entering ratios of their sizes in an AHP pairwise comparison matrix. It leads to a valid answer and he explains that ratios are also used when comparing elements on an intangible property. It is a validation exercise. To validate a process you use it to see if gives you the right answer when you know the answer. If it does, then you can feel more confident that it will work when you don't know the answer.