The position of test data in frequency distribution can be expressed by percentage grade. Percent grade reflects the percentage of the total number of data below an observation score, and a value between 0~ 100 is taken as the norm of personality or educational test.

(5) "standard score" ("isometric scale")

"Standard score" is converted from "original score". The scale that expresses the distance between "original score" and "average score" in units of "standard deviation" is called "isometric scale". Standard score is an isometric scale. The isometric drawing ruler not only has a size relationship, but also has equal units, so it can be added and subtracted, but there is no absolute zero, so it cannot be multiplied and divided.

Equidistant scales have the same number unit, and the difference between two adjacent numbers is equidistant. If the temperature is 60 degrees to 40 degrees, then the difference between 50 degrees and 30 degrees is equal. This scale can mean "more or less", but it does not mean multiple relationships. For example, the temperature of "0" is not absolute, it is decided by people, so it cannot be said that 15 is only half as hot as 30 degrees. In education and psychological measurement, the quantitative rule of isometric scale is that the arithmetic between quantity must be able to express the arithmetic of psychology. Because a large number of statistical methods are suitable for isometric scale, there are two main methods commonly used in education and psychological measurement to realize the conversion of evaluation scale to isometric scale: one is to convert the original score into standard score, and standard score is an isometric scale. Secondly, the isometric scale can be used on the premise that the normal distribution and the assumed standard deviation are equal.