Independent variables are the variables that are manipulated to then analyze the effect on the dependent variable. Although, there is not necessarily manipulation of the independent variable but allows for breaking them into groups that fit into specific properties for the study. An example I found to explain this a little clearer is a study of white cell count between the two genders. There is not a manipulation, but the independent variables are male and female and the dependent variable (what is measured) is the white blood cell counts of the genders (StatSoft Electronic Statistics Textbook, 2012).
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