15.32. Astudy13 compared the relative frequency of mental health problems of various types among U.S. Army members before deployment to Iraq,U.S. Army members after serving in Iraq, U.S. Army members after serving in Afghanistan, and U.S. Marines after serving in Iraq. The study stated, “Potential differences in demographic factors among the four study groups were controlled for in our analysis with the use of logistic regression.” For this study, identify the response variable, the primary explanatory variable, and likely control variables.