The numerical observations or scores obtained from measuring some definable attribute of a set of objects are at the interval level of measurement if the following three conditions exist:
- The order of the numbers corresponds to the rank order of the objects with respect to the attribute being measured.
- The difference between any two consecutive numbers on the measurement scale is the same regardless of which pair of adjacent numbers is considered.
- The zero point of the number scale used represents an arbitrary origin and does not indicate complete absence of the property being measured.
Scales that satisfy all three conditions are also described as equal-interval scales or equal-unit scores. The diagram in Figure 1 graphically displays a segment excerpted from an equal-interval scale. The letters a, b, c, ...