On interval measurement scales, one unit on the scale represents the same magnitude
on the trait or characteristic being measured across the whole range of
the scale. For example, if anxiety were measured on an interval scale,
then a difference between a score of 10 and a score of 11 would represent
the same difference in anxiety as would a difference between a score of 50
and a score of 51. Interval scales do not have a "true" zero point, however,
and therefore it is not possible to make statements about how many times
higher one score is than another. For the anxiety scale, it would not be
valid to say that a person with a score of 30 was twice as anxious as a
person with a score of 15. True interval measurement is somewhere between
rare and nonexistent in the behavioral sciences. No interval-level scale
of anxiety such as the one described in the example actually exists. A
good example of an interval scale is the Fahrenheit scale for temperature.
Equal differences on this scale represent equal differences in temperature,
but a temperature of 30 degrees is not twice as warm as one of 15 degrees.