In order to find a relationship between stimulus and perception we need to quantify perceptions. To quantify perceptions we need to know a little about numbers. There are four different types of number scales that can be used to measure perception, some providing more information than others. The simplest of scales is the nominal/categorical scale. With this scale, numbers are used only as labels, for example, the numbering of football players. A second scale is the ordinal scale. The ordinal scale is used to rank stimuli greater than or less than each other, for example, the rating of the pleasantness of odors. With the this scale, the intervals between the assigned numbers are meaningless. A third scale is the interval scale. The interval scale is concerned with equality of intervals/differences between assigned numbers. An example of an interval scale is temperature scales in Fahrenheit or Celsius. The final scale is the ratio scale, which too has meaningful intervals.  The difference between this scale and the interval scale is that the ratio scale has an absolute zero, as is the case when measuring length or weight or density (Stevens, 1951).