Which of the following is NOT a type of variable in research methods?

In research methods, dependent, independent, and controlled/constant variables are well-established concepts that describe the relationships between factors being studied.

The dependent variable is the outcome or effect that researchers are interested in measuring, while the independent variable is the factor that is manipulated to observe its effect on the dependent variable. Controlled or constant variables are those that researchers keep the same throughout the study to ensure that any changes in the dependent variable can be attributed to the independent variable, minimizing confounding effects.

On the other hand, residual does not refer to a type of variable in the same context. Instead, it typically relates to the difference between the observed values and the values predicted by a model. This is particularly relevant in regression analysis, where the residuals are the errors of prediction rather than a distinct type of variable like the others mentioned. Thus, identifying "residual" as not being a type of variable aligns with foundational concepts in research methodology, differentiating it from established variables that play key roles in experimental design and analysis.