Experiment 48

Stepwise regression for statistical control
Partial Correlation

In the age range 16 to 50 years there is a known correlation between the age of a patient and the time it takes to recover from a depressive episode. Older patients recover faster but it is also known that they tend to have less severe depressive episodes. As recovery time can be expected to be shorter for less severe episodes, it is decided to look at the effect of age on recovery time while statistically controlling the severity of the depressive episode. Accordingly all the patients attending an out-patients clinic to be treated for depression are assessed using a standard measure of depression each time they visit. Their initial depression score is taken as a measure of the severity of the depressive episode. A criterion for recovery is set using the same scale and the time it takes to reach that criterion is recorded. All 53 patients reach the criterion within 90 days. The age of each patient was obtained from medical records.

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