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.
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About Thomas L. Saaty
He was a Distinguished University Professor at the University of Pittsburgh.
In 2000 he received the Gold Medal for his work on decision making from the International Society for Multi-criteria Decision Making.
In 2007 he received the Akao prize given by the Quality Function Deployment (QFD) society.
In 2008 he received the Impact Prize from INFORMS (the society for Operations Research) for his seminal work on the Analytic Hierarchy Process and for his contribution to its deployment and its extraordinary impact.
In 2009 he received the Chancellor's Distinguished Research Award at the University of Pittsburgh.
