A weight-lifting trainer wants to determine if a new protein shake helps weight lifters bench-press more weight. To find out, she interviews weight lifters at a local fitness center, asking them how much they can bench-press and whether they drink the shake. She finds out that the lifters who drink the shake can bench-press more, so she concludes that the shake works.
a. Describe the problems with the trainer’s study. Include in your description at least one possible lurking or confounding variable and identify which type it is.
b. Redesign the study so that it is a well-designed experiment.