In statistics, there are two types of variables:
Quantitative Variables: Variables that represent a measurable quantity. Examples include:
- Square footage
- Population size
Qualitative Variables: Variables that take on names or labels and fit into categories. Examples include:
- Eye color
- Marital Status
- Dog breed
One question that students often have is:
Is age considered a qualitative or quantitative variable?
The short answer:
Age is a quantitative variable because it represents a measurable quantity.
For example, if someone is 35 years old then we know they are 5 years older than a 30 year-old but 5 years younger than a 40-year old.
We couldn’t say the same thing about a qualitative variable like “eye color” because it doesn’t make sense to compare “blue” vs “green” vs “brown” eyes in numerical terms.
Also, since age is a quantitative variable this means we can calculate summary statistics for it, such as:
- Measures of central tendency like the mean, median, and mode.
- Measures of dispersion like the range, interquartile range, and standard deviation.
For example, if we have a dataset that contains the ages of 100 individuals, we could calculate the mean age, median age, the range of ages, and so on.
We could not do the same for qualitative variables.
When Is Age Not a Quantitative Variable?
The only scenario where age would not be considered a quantitative variable is when we use age brackets.
In this scenario, age would be a qualitative variable because ages could be grouped into categories.
For example, suppose an economist wants to study the relationship between annual income and age so he sends out a survey to 1,000 individuals and asks them to indicate their age bracket as one of the following:
- Under 18
- 18 to 35
- 35 to 52
- 53 to 70
- Over 70
In this scenario, age would be considered a qualitative variable because each individual would fall into a certain age category as opposed to a specific numerical age value.
In this scenario, we couldn’t calculate summary statistics like the mean and median because we don’t actually know the specific age of each individual.
The following tutorials offer additional information about types of variables: