Variables at a ratio scale of measurement are, typically, those such as age, number of children, etc. The values of these variables have a real "zero" value.
For example, a person with 4 children has exactly twice as many children than a person with only two; or a person who is 25 years old is exactly 10 years older than a person who is 15 years old.
Furthermore, there is no person with -2 children or who has -20 years of age.
The values of variables at an interval scale are quantified numerically. Because there is a real zero value, this measurement does not go below its real zero value.
We may use the concept of "ratio variable" (or "ratio-level variable") to refer to those variables which use an interval scale of measurement. This is "handy" although not fully correct. That is, the quality of being "ratio" refers to the scale not to the variable. That is, it is possible to transform the values of a ratio variable into another level of measurement.
- Ratio variables vary quantitatively in a fixed and dependable manner, and have a real zero value of reference.
- If required, the most appropriate "average" for ratio variables is the mean.
Want to know more?
- Wikipedia - Level of measurement
- You can learn a bit more about levels of measurement in Wikipedia.