There are four measurement scales (or types of data) to categorize different types of variables: nominal, ordinal, interval and ratio.

Interval data has not only the properties of ordinal data but also “+” and “-”. Interval scales are numeric scales in which we know not only the order, but also the exact differences between the values. Interval scales do not a “true zero”, below that there can be negative values. Hence, you cannot use “×” or “÷” to describe the relationship of two values. The classic example of an interval scale is Celsius temperature. The difference between 40 and 10 degrees is a measurable 30 degrees, yet 40 degrees is not four times as 10 degrees. 0 degree is not a true zero, below that there are -10 degree , -20 degree, etc.