2.40. An interval scale for which ratios are valid is called. a ratio scale. Such scales have a well-defined 0. point, so, for instance, one can regard the value 201 as twice the quantity of the value 10. Explain why annual income is measured on a ratio scale, but temperature (in Fahrenheit or Centigrade) is not. Is IQ, as a measure of intelligence, a ratio-scale. variable?