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In quantitative research, assessing the cause or causes that explain a correlation usually involves distinguishing independent from dependent variables. An independent variable is one that produces an effect on another variable. The variable affected is called the dependent variable. In the example above, academic achievement is the independent variable and occupational income the dependent variable. The distinction refers to the direction of the causal relation. However, the same factor may be an independent variable in one study and a dependent variable in another. It depends on what causal processes are being analysed. If we were looking at the effects of differences in occupational income on people’s lifestyles, then occupational income would be the independent rather than the dependent variable.

To find out whether a correlation between variables is a causal connection, we can use controls, which means we hold some variables constant in order to look at the effects of others. By doing this, we are able to judge between explanations of observed correlations, separating causal from non-causal relationships. For example, medical researchers studying smoking behaviour may suggest that vaping via e-cigarettes leads to a reduction in conventional tobacco smoking in young adults aged eighteen to twenty-four – that there is a causal connection between e-cigarette use and smoking cessation. To find out, we could gather a sample of conventional smokers in this age group and assign them randomly into two groups: an experimental group that is given e-cigarettes and a control group that is not. After the study period, we would measure tobacco-smoking activity in the two groups; if the experimental group smokes less than the control group after their experience with e-cigarettes, we may attribute the reduction to the latter.

This is a simplified example to illustrate the principle of using control groups in variable analysis, but in reality things can be more messy and complex than is suggested here. Was it really vaping that caused the reduction in tobacco smoking? Could it be that, during the research process, positive praise for trying to stop smoking received from friends and family was the strongest factor?