Scaling refers to assigning numerical values to psychological attributes. Individuals in a group should be similar to each other in the regard of sharing a psychological feature. There are rules to follow in order to put people in categories:

1. People in a category must be identical with respect to the feature that categorizes the group (e.g. hair colour).
2. The groups must be mutually exclusive
3. The groups must be exhaustive (e.g. everyone in the population can fall into a category).

Each person should fall into one category and not more than one. If numerals are used to indicate order, then the numerals serve as labels indicating rank. If numerals have the property of quantity, then they convey information about the exact amounts of an attribute. Units of measurement are standardized quantities. The three levels of groups are identity (1), order (2) and quantity (3).

There are two possible meanings of the number zero. It can be the absolute zero (1) (e.g. a reaction time of 0ms) or it can be an arbitrary quantity of an attribute (2). This is called the arbitrary zero. The arbitrary zero does not represent the absence of anything, rather, it is a point on a scale to measure that feature. A lot of psychological attributes use the arbitrary zero (e.g. social skill, self-esteem, intelligence).

An unit of measurement might be arbitrary because unit size may be arbitrary (1), some units of measurement are not tied to any one type of object (2) (e.g. centimetres can measure anything with a spatial property) and some units of measurement can be used to measure different features of the same object (3) (e.g. weight and length).

One assumption of counting is additivity. This requires that unit size does not change. This would mean that an increase of one point is equal at every point. This is not always the case, as an IQ test asks increasingly difficult questions to increase one point of IQ. Therefore, the unit size changes.

Counting only qualifies as measurement if it reflects the amount of some feature or attribute of an object. There are four scales of measurement:

1. Nominal scale
   This is used to identify groups of people who share a common attribute that is not shared by people in other groups (e.g. ‘0’ for male and ‘1’ for female). It assesses the principle of identity.
2. Ordinal scale
   This is used to rank people according to some attribute. It is used to make rankings within groups and cannot be used to make comparisons between groups, as this would require quantity. It assesses the principle of identity and order.
3. Interval scale
   This is a scale that is used to represent quantitative difference between people. It assesses the principle of identity, order and quantity.
4. Ratio scales
   This is a scale that has an absolute zero point. It satisfies the principle of identity, order, quantity and has an absolute zero.

Psychological attributes might not be able to be put on a ratio scale, as the measurements might have an absolute zero, but many psychological attributes do not.