Variables are a fundamental element in research, as they allow us to describe, analyze, and interpret the phenomena under study. There are different ways to classify them; however, one of the most commonly used distinctions is between qualitative and quantitative variables

In this lesson, we will discuss one of the many ways to classify variables; we will focus on qualitative and quantitative variables. Understanding the differences between each type of variable will allow for proper analysis and interpretation, leading to real and useful research results

Let’s start with qualitative variables. Qualitative variables refer to those that are measured in categories. An important characteristic of this type of variable is that the categories must be mutually exclusive; this means that an individual or unit of analysis cannot belong to two categories at the same time

These variables can be divided, according to the number of categories they have, into dichotomous or polytomous. A dichotomous variable is one that cannot have more than two categories (yes or no, zero or one). On the other hand, polytomous variables are those that have more than two categories (good, fair, or poor)

Qualitative variables can be measured on nominal or ordinal scales.¹ A nominal qualitative variable is one that does not have a specific order; an example of this would be the variable name or sex. Male or female sex has no particular order, just like people’s names. On the other hand, ordinal qualitative variables can be organized in a logical way that allows for identifying a gradient among the categories; an example of this is the stages of life (childhood, adolescence, adulthood, and old age).

Quantitative variables are those that can be expressed numerically. These can be divided into discrete or continuous quantitative variables.¹˒² Discrete quantitative variables are those that can only take predefined values and do not have intermediate values between them. An example is the number of children a mother can have (she can have one, two, or more children, but never 1.5 or 2.4 children).

Continuous quantitative variables are those that can take any value within a logical range. An example is a baby’s weight measured in kilograms, where values can range from 1.952 kg to 10.653 kg

An important characteristic of quantitative variables is that, depending on their level of measurement, they can be added, subtracted, multiplied, or divided while maintaining a logical meaning in the results of these operations. The opposite occurs with qualitative variables, where attempting such operations can produce meaningless results. For example, the identification numbers of two people (15234 and 07854), although composed of numbers, actually function as labels rather than numerical values; therefore, if one tried to add or subtract these values, the result would have no logical meaning.

• Variables can be divided into qualitative variables (which cannot be expressed numerically) and quantitative variables (which can be presented numerically).
• Qualitative variables can be divided according to the number of categories into dichotomous (having only two categories) or polytomous (more than two categories).
• Quantitative variables can be divided into discrete variables (which can only take predefined values) or continuous variables (which can take any value within a range).
• Quantitative variables can be added, subtracted, multiplied, or divided depending on their level of measurement.

• ¿Qué es una variable? – epidemios.com

1. Kaliyadan F, Kulkarni V. Types of variables, descriptive statistics, and sample size. Indian Dermatol Online J 2019;10:82-6.
2. Villasís-Keever MA, Miranda-Novales MG. El protocolo de investigación IV: las variables de estudio. Rev Alerg Mex. 2016;63(3):303-310

Gerhard M Acero
ESP epidemiology
Publication Date: 01/02/2026

This post has not been sponsored