Unlike the other pages in this guide, independent and dependent variables do not refer to types of studies, but components of studies.
Variables are any characteristics in the study that can take on different values. The main difference between independent and dependent variables is cause and effect. The independent variable is not expected to be impacted by the study (it's independent), but to cause the difference in the dependent variable. The dependent variable is the effect. The dependent variable expected to change because of the independent variable (it depends on the other factors involved).
Is this a variable that the researchers deliberately introduced or that would have occurred regardless of the study?
The independent variable is the cause, not the effect. So if researchers introduce something in the experiment, like an intervention, that's the independent variable. For observational studies, the independent variable is what was already present in the patients before the outcome that's being measured.
Examples
An observational study wants to know if patients who worked high stress jobs had more strokes. Having a high stress job is the independent variable. It's not really the variable that's being measured. It's the variable that may or may not cause strokes.
An experimental study wants to know if training soccer players on knee stability exercises reduces the number of injuries in a season. The knee stability training is the independent variable. Here, the researchers deliberately introduced training on knee stability exercises. It's not what they want to measure; they want to measure injuries. But this variable that they've introduced is what may or may not cause a reduction in injuries.
Is this the variable that is being studied/measured?
The easiest way to know what is the dependent variable is to look at what the study is trying to measure. That's the dependent variable, it's what the researchers expect will be impacted by other factors in the study, it's the factor that they're wanting to measure.
If this is an experimental study, is this the variable that would be impacted by the intervention?
The dependent variable depends on the other variables. It is the thing that will be affected by the other variables in the study.
Examples
An observational study wants to know if patients who worked high stress jobs had more strokes. Having or not having a stroke is the dependent variable.
An experimental study wants to know if training soccer players on knee stability exercises reduces the number of injuries in a season. The number of injuries in the season is the dependent variable.