Ratio data

The highest level of numeric data is ratio scale. With ratio data, you can make meaningful comparisons using division and subtraction. It is meaningful to say that you have acquired twice as many customers when you went from 50 to 100 customers in one month. Ratio data also adds 0 to the domain of numbers (which interval data does not contain), so it is meaningful to say that you have 0 customers (although not desireable). Weight and income are other examples of ratio data. however, the fahrenheit and celsius temperature scale do not have a 0 meaning no temperature. That creates some interesting comparisons if you would try to make temperature comparisons. For example, in fahrenheit, a temperature of 36/18 equals 2, but the equivalent celsius is 2.22/-7.78 which equals 0.28 which suggests that ration comparisons are meaningless, and that both sides should be treated as interval. 

To learn more about the theory of scales and measurement refer to On the Theory of Scales and of Measurement (Stevens, 1946).