The process of measurement involves assigning numbers to observations according to rules. The way that the numbers are assigned determines the scale of measurement. Four scales of measurement are typically discussed in psychological statistics.

• nominal
• ordinal
• interval
• ratio

We will add a fifth scale of measurement.

• approximately interval

Each scale of measurement represents a particular property or set of properties of the abstract number system. The mathematical properties of the numbers you are going to analyze are important because they determine which statistics you can use.