The variables in a research study are all that is measured, the information collected, or the data that is collected in order to answer the research questions, which are specified in the objectives. Their selection is essential to the research protocol. This article aims to point out the elements to be considered in the section of the variables. To avoid ambiguity, it is necessary to select only those that will help achieve the study objectives. It should subsequently be defined how they will be measured to ensure that the findings can be replicated; it is therefore desirable to include conceptual and operational definitions. From the methodological point of view, the classification of variables helps us understand how the relationship between them is conceptualized. Depending on the study design, the independent, dependent, universal, and confounding variables should be noted. Another indispensable element for planning statistical analyses is the scale of variable measurement. Therefore, one must specify whether the variables correspond to one of the following four: qualitative nominal, qualitative ordinal, quantitative range, or quantitative ratio. Finally, we should detail the measurement units of each variable.