Level of measurement

One of the most important properties of variables is the level of measurement, also called scales of measurement. The measurement scale is important because it determines the permissible arithmetic operations and thus specifies the possible statistical tests. The higher the level of measurement, the more comparative statements and arithmetic operations are possible.

The level of measurement of a variable can be either nominal, ordinal or metric. In a nutshell: For nominal variables the values can be differentiated, for ordinal variables the values can be sorted and for metric scale level the distances between the values can be calculated. Nominal and ordinal variables are also called categorical variables

Nominal variables

The nominal measurement scale is the lowest level of measurement in statistics and therefore has the lowest information content. Possible values of the variables can be distinguished, but a meaningful order is not possible. If there are only two characteristics, such as gender (male and female), we also speak of dichotomous or binary variables.

• Only relations "equal", "unequal" possible
• No logical ranking of categories
• The order of the answer categories is interchangeable
• Nominal characteristics with only two expressions are also called "binary" or "dichotomous"

Ordinal variables

The ordinal level of measurement is the next higher level, it contains nominal information, only with the difference that a ranking can be formed, therefore the term ranking scale is often used. In these cases, however, the distances between the values are not interpretable, so it is not possible to make a statement about the absolute distance between two values. A classic representative of the ordinal scale are school grades, here a ranking can be formed, but it cannot be said that the distance between A and B is the same as the distance between B and C.

• Next higher scale of measurement
• "equal" and "unequal" or "greater" and "smaller" can be determined
• There is a logical hierarchy of categories
• The distances between the numerical values are not equal, i.e. cannot be interpreted

Categorical variables

Variables that have a nominal scale or an ordinal scale are also called categorical variables. In other words, categorical is an umbrella term for variables scaled nominally and ordinally.

Categorical variables can have a limited and usually fixed number of expressions, e.g. country with Germany, Austria, ... or gender with female and male. It is important, however, that it must be a finite number of categories or groups. The different categories can have a ranking, but do not have to.

Metric variables

Metric variables have the highest possible level of measurement. With a metric level of measurement, the characteristic values can be compared and sorted and distances between the values can be calculated. Examples would be the weight and age of subjects.

• Highest level of measurement
• Creation of rankings possible
• "equal" and "unequal", "greater" and "smaller" can also be determined
• Differences and sums can be formed meaningfully

Ratio scale and interval scale

The metric level of measurement can be further subdivided into interval scale and ratio scale. As the name suggests, the values of the ratio scale can be put into a ratio. Thus, a statement like the following can be made: "One value is twice as large as another". For this, an absolute zero must be available as a reference.

The classic example is the temperature indication in degrees Celsius and Kelvin. The zero point of the Kelvin temperature scale is absolute zero, therefore it is a ratio scale. At degrees Celsius the absolute zero point is -273.15 °C, therefore the value zero on the degree Celsius scalar cannot be assumed as natural zero and therefore it is an interval scale

Scale level examples