Independent and Dependent Variables

An independent variable is the variable (or variables) you’re interested in testing for an effect.  It often organizes the data into groups or is a variable you want to look for a relationship to some other variable. If  one variable is a “cause”, it is your independent variable.

A dependent variable is a variable you are looking for an effect on, or a relationship with.  If you are looking for an effect, it is likely your dependent variable.

Despite this, in a single study, variables may play both independent and dependent roles in different analyses.  For example, in a study looking at effect of patient weight on operative times and surgical outcomes, patient weight would be an independent variable in testing for a relationship on operative time (which would be dependent).  You are testing the effect of weight (independent) on surgical time (dependent).  If you then ran a regression looking at both the effect of weight and time on post-operative complications, then both weight and surgical time are independent in this test and complication rate is the dependent variable.