What can interval variables typically be measured in?

Interval variables are characterized by having both order and a consistent scale between values. This means that the difference between any two values is meaningful and equal across the scale. As a result, interval variables can be measured in equal intervals. An example of this would be temperature measured in degrees Celsius or Fahrenheit; the difference between 20 and 30 degrees is the same as that between 40 and 50 degrees, illustrating the concept of equal intervals.

In contrast, categories represent nominal data without a meaningful order, ranks imply a relative position without informing about the actual distance between ranks, and proportions relate to parts of a whole but do not maintain the consistent interval property essential for interval measurements. Therefore, the defining characteristic of interval variables being governed by equal intervals makes the choice of equal intervals the correct answer.