Interval measurement scales
The next metric scale is called interval measurement scales. Interval along with ratio scales affords the most precise data. With both there are equal intervals or units between any two consecutive numbers.
The only difference between interval and ratio scales is the role of zero (0). In interval scales zero is artificial rather than real. IQ and standardized tests for performance assessment have a zero (0), but it is artificially created rather than real. In ratio scales, zero is real. It signifies the absence of the variable under study. An example is Number of Days Absent from School.
Interval scales have a zero (0) point in them, but it is arbitrary. Good examples are the interval scales used for temperature  Fahrenheit and Celsius. The difference between 15° Celsius and 20 ° Celsius is 5°. The same is true for Fahrenheit. But there is no absence of temperature. The zero is an arbitrarily point in both scales. The same is true for IQ scales.
Return from interval measurement scales to variables and instruments.
