Which of the following is the best explanation for the fact that the graduate admissions rate is lower for women than for men?

i. The admissions process discriminates against women.

ii. Women tend to apply to departments that are harder to get into.
The data set in this case study provides an example of a result known as Simpson’s Paradox. Simpson’s Paradox occurs when two variables in a contingency table are dependent, but the dependence vanishes or reverses when separate tables are made for each level of a confounder. The lesson to be learned is that lack of independence is not the same as causation. When we reject the null hypothesis of independence in an observational study, it is possible that the result may be due to confounding. To determine whether a certain variable is a confounder, it is necessary to make a separate table for each value of the confounder.