When a researcher asks the question, Was the subject dropout rate during the course of the study equal over the various conditions or more pronounced in certain treatment groups?, she is concerned with which one of the following potential confounds
a. regression
b. random error
c. mortality
d. events outside the laboratory (history)
Question 2
If people who drop out of study are systematically different from those who stay, we have which threat to validity?
a. selection
b. mortality
c. regression to the mean
d. maturation
