In statistics, all variables are measured on one of four measurement scales:

**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 “time” considered an interval or ratio variable?**

The short answer:

**Time is considered an interval variable because differences between all time points are equal but there is no “true zero” value for time.**

For example, the difference between 1 PM and 2 PM is the same as the difference between 2 PM and 3 PM, which is the same as the difference between 3 PM and 4 PM, and so on.

However, there is not “true zero” value of time. For example, we can’t say that 2 PM is twice as old of a time as 1 PM.

Contrast this with a ratio variable like weight: We can say that 100 pounds is twice as much as 50 pounds. The same cannot be said for time.

**When is Time Not an Interval Variable?**

The only scenario where time would not be considered an interval variable is if we’re talking about a **duration of time**.

Consider the following scenarios:

**Scenario 1: Marathon Times**

Suppose we keep track of how long it takes people to run a marathon. In this scenario, the duration of time would be considered a ratio variable because there is a “true zero” value – zero seconds.

We could also say that someone who runs the marathon in 2 hours ran it in half the amount of time as someone who ran it in 4 hours.

**Scenario 2: Cooking Times**

Suppose we compare two recipes for cooking a certain meal. One recipe has a total cooking time of 40 minutes and the other has a cooking time of 20 minutes.

In this scenario, the duration of cooking time would be considered a ratio variable because there is a true zero value – zero minutes.

We could also say that one recipe has a cooking time that is twice as long as the other.

These represent scenarios where we would classify time as a ratio variable instead of an interval variable.

**Additional Resources**

The following tutorials offer additional information on types of variables:

Levels of Measurement: Nominal, Ordinal, Interval and Ratio

Is Age a Discrete or Continuous Variable?

Categorical vs. Quantitative Variables