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Relative validity

Relative validity is the extent to which two methods of measurement can rank individuals in the same order, capturing their relative differences but not absolute differences. For example, you could test the relative validity of two different measurements of water consumption in a day. Say one measure asked for the number of glasses of water drunk per day and the other measure asked for the number of litres of water drunk per day. It is not possible to directly compare these two measures as they use different units – one uses glasses (and doesn’t specify the size of the glass) and one uses litres. If both measures ranked all the individuals in the same order (i.e. the order of those drinking the most to least amount of water was the same in both measures), then they would show high relative validity. By contrast, absolute validity refers to the degree of agreement between two methods measuring the same phenomenon in the exact same units.