Generally data can be described as measured or categorised.
Measured data take numerical values on a scale with equal units (such as centimetres, minutes or dollars).
Because they have an associated scale, measured data can have arithmetic operations performed on them.
For example, if the total reaction time for 30 students was 15.9 seconds, then the mean reaction time can be calculated as 0.53 seconds.
In categorised data, each observation falls into one of a number of distinct categories (such as colour preference or gender).
Categorised, or categorical data, cannot have arithmetic operations performed on them. They are usually summarised with counts or percentages. Even if the data are associated with a number, arithmetic operations cannot be performed.
For example, if data are collected on the year level (7–10) of 30 students, the mean year level cannot be calculated. One cannot say that the average year level is 8.3 but rather that 20% of the group are in year 7.
