3. Ordinal scales of measurement
Arithmetic operations that can be performed on values assigned to ordinal scales are also limited. As well as counting the numbers in each category on the scale it is possible to indicate relative size using equals (=), greater than (>) or less than (<) symbols.
Any operation involving addition and subtraction is not allowed - for example calculating the mean. Calculating the median and mode as measures of central tendency is appropriate as these only involve counting the numbers in each category on the scale.