As part of his dissertation, Joe wanted to know if an ice cream reward can increase the number of books read by children in the first grade. The small school has four 1^st grade classes - two are given the reward and two are not; the total number of books read at the end of the term are compared.

What is the independent variable? Is it a true manipulated variable or a subject/participant variable?

What is the dependent variable? What is its scale of measurement?

He found a huge difference - students read more in the classes rewarded with ice cream. What is a potential confounding variable?