The ratio scale provides all the information that the interval scale does. In addition, the ratio scale provides an absolute zero point. That is, when you reach zero, you do not have any of the variable left.
Absolute Zero means that a score of zero indicates a complete absence of the characteristic being studied.
Annual household income (measured in dollars) is a good example of a common ratio level variable. When you earned zero dollars last year, then you have no income.
Contrast this with temperature measured on a Fahrenheit thermometer. A measurement of zero does not mean that you have no heat left. You can keep going below zero. A Kelvin thermometer has its zero point at absolute zero. Nothing can be colder than zero degrees Kelvin. Thus, temperature, when measured on a Fahrenheit thermometer is on an interval scale. Temperature, when measured on a Kelvin thermometer, is on a ratio scale.
The Ratio Scale orders and measures data, as well as having an absolute zero.
Note that there is very little difference between the interval scale and the ratio scale. Most statistics that are appropriate for one are appropriate for the other. This is why you will often see both categories with a slash between them in the methodology literature.
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Last Modified: 06/29/2018