Ideally, in an experimental design you want to control all but one variable. In real-life human studies, you can almost never do this exactly, but you try to ap

Ideally, in an experimental design you want to control all but one variable. In real-life human studies, you can almost never do this exactly, but you try to approximate a true controlled experiment as much as possible. In this case, the independent variable is cocaine exposure, and full-term birth is a controlled variable.