Level of measurement

Level of measurement refers to how we classify and organize data. It helps us understand what kind of information the data provides and determines the types of calculations and analyses we can perform.

There are three main levels: 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. Metric variables can be further divided into interval variables and ratio variables.

Therefore, levels of measurement refer to the different ways that variables can be quantified or categorized. If you have a data set, then every variable in the data set corresponds to one of the three (or four) primary levels of measurement.

In practice interval and ratio data are often used to perform the same analyses. Therefore, the term 'metric level' is used to combine these two levels.

Why You Need Levels of measurement

The level of measurement is crucial in statistics for several key reasons. It tells us how our data can be collected, analyzed, and interpreted.

Different levels of measurement support different statistical analyses. For instance, mean and standard deviation are suitable for metric data. In some cases, it may be suitable for ordinal data, but only if you know how to interpret the results correctly. And it definitely makes no sense to calculate it for nominal data.

The level of measurement also tells us which hypothesis tests are possible and determines the most effective type of data visualization.

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 in the case of gender (male and female), we also speak of dichotomous or binary variables.

• Only relations "equal" and "unequal" are 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 also contains nominal information, only with the difference that a ranking can be formed. The term ranking scale is hence often used. In these cases, 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", as well as "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 may have a ranking, but must not.

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", as well as "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 scale cannot be assumed as natural zero and therefore it is an interval scale.

Scale level examples