Are there side effects of watching 3D movies? More specifically, is nausea a side effect? A study done in Italy (Solimini, 2013) had moviegoers rate their levels of nausea on a 7-point scale after watching a movie at a theater. For the 433 that watched 2D movies, of which Sherlock Holmes: A Game of Shadows was the most popular, 21.5% of them reported having at least some nausea. For the 451 that watched 3D movies, of which the cartoon Puss in Boots was the most popular, 46.1% of them reported having at least some nausea. The most popular time to see the 2D movies was late evening, while the most popular time to see the 3D movies was early evening. 

a. Identify the observational units and how many there are. 

b. Identify the explanatory and response variables. Also identify the type (categorical or quantitative) of each. 

c. Based on the conditional proportions given, does it appear there is an association between the movie type (2D versus 3D) and having at least some nausea? Explain. 

d. What are some potential confounding variables in this study?