INDEPENDENT VARIABLE

The INDEPENDENT (sometimes known as PREDICTOR) variable is one that, in an experimental study, the experimenter manipulates, for example by randomly assigning participants to a therapy or control group. The DEPENDENT variable - in this example therapeutic change, is expected to DEPEND on which condition the participant is assigned to. Some more examples follow below.

EXAMPLE 1

If you had a sample of adult men and women, and you measured their heights in millimetres, you would probably find that the mean male height was slightly greater than the mean female height. You might call gender the INDEPENDENT variable and height the DEPENDENT variable. The person's gender is INDEPENDENT of any other variable. Gender can also be called the PREDICTOR variable here.

EXAMPLE 2

Here is a more complex case: say you are predicting the satisfaction ratings given by clients about a therapy from their scores on a pre-therapy expectations questionnaire, a measure of strength of therapeutic alliance established, and the level of change they reported on a key issue that was tackled in the therapy. In this case, satisfaction is your DEPENDENT variable. You could say that its value DEPENDS on the values of the predictor or INDEPENDENT variables (prior expectations, therapeutic alliance, amount of change). 

NB: Correlation does not necessarily imply causation. In example 2 there would be a degree of CORRELATION between the predictors (independent variables) and dependent variable. However, it would be incorrect to assume that therefore the independent variables CAUSE the variation in the dependent variable. It might be that any or all of realistic expectations, a good therapeutic alliance and substantial change cause satisfaction, but equally it could be that high motivation and interest in that therapy lead to realistic expectations, help the establishment of a good alliance, foster positive change, and also predispose a client to report satisfaction. Correlation can be taken as evidence that there may be a causal link, but it is not conclusive evidence.

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