For more than a century, psychologists have studied the processes by which internal states are mapped onto overt responses. An important distinction drawn in this research is between category scales and magnitude scales. Category scales consist of a bounded set
of ordered responses, as in the familiar format of many opinion surveys. The categories can be represented by numbers; in such cases descriptive labels are always attached to the extremes of the scale, and sometimes to some or all intermediate values. It is generally
assumed that category scales are invariant to a linear transformation: a response of 5 on a scale that ranges from 1 to 7 is interpreted as equivalent to a response of 40 on a scale that ranges from 0 to 60. As this example illustrates, the zero point on a category scale has
no particular significance