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Age is a ratio variable because it is measured on a ratio scale. Ratio variables have a meaningful zero point, meaning the difference between two values is meaningful; for example, the difference between a person who is 25 years old and a person who is 50 years old is 25 years. This is not true for interval variables, where the difference between two values is not meaningful. For example, the difference between two temperatures of 72 degrees and 77 degrees is 5 degrees, but that does not mean the temperature is 5 times warmer.
In statistics, all variables are measured on one of four :
- Nominal: Variables that have no quantitative values.
- Ordinal: Variables that have a natural order, but no quantifiable difference between values.
- Interval: Variables that have a natural order and a quantifiable difference between values, but no “true zero” value.
- Ratio: Variables that have a natural order, a quantifiable difference between values, and a “true zero” value.
The following graphic summarizes these different levels of measurement:
One question students often have is:
Is “age” considered an interval or ratio variable?
The short answer:
Age is considered a ratio variable because it has a “true zero” value.
It’s possible for an individual to be zero years old (a newborn) and we can say that the difference between 0 years and 10 years is the same as the difference between 10 years and 20 years.
Since age is a ratio variable, we can also say that someone who is 10 years old is twice as old as someone who is 5 years old.
Contrast this with an interval variable like temperature: We cannot say that 10 degrees Celsius is twice as warm as 5 degrees Celsius because there is no “true zero” when it comes to temperature since degrees can be negative.
When is Age Not a Ratio Variable?
The only time that age would not be considered a ratio variable is if the data we collect on age is in categories.
For example, we may send out a survey and ask people to report which age bracket they belong in from the following choices:
- 0-19 years old
- 20-39 years old
- 40-59 years old
- 60+ years old
In this scenario, age would be treated as an ordinal variable because a natural order exists among the potential values.
We would say 0-19 years old is younger than 20-39 years old, which is younger than 40-50 years old, which is younger than 60+ years old.
This represents a rare scenario where we would not classify age as a ratio variable.