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A ratio scale of measurement is one based on a fixed zero point and where differences between values can be quantified in absolute terms.
A good example is age where equal differences between scores represent equal differences between the thing being measured. So that two years between the age of 2 and 4 is the same length as between 32 and 34. It is because of these mathematical properties that it makes sense to talk about someone being twice as old as someone else.
An absolute scale is a ratio scale in which the unit of measurement is fixed