Athletes routinely swipe black grease under their eyes to help cut down on glare on sunny days. Recently, some athletes have switched from grease to patches of black tape. Does either method work? Which is best? A recent study helped to answer these questions. A sample of 46 subjects were tested using the Pelli-Robson contrast chart, which gives a numerical rating for ability to discern contrast against a sunlit background. Subjects were then randomly assigned to one of three groups and tested again. One group used black grease, one used black tape patches, and one used clear petroleum jelly. The group wearing the black grease was the only group to show significant improvement in discerning contrast in sunlight.

(a) Is this an experiment or an observational study? Explain.

(b) Why is this study not double-blind (or even single-blind)?

(c) What is the sample in this study? Give a reasonable intended population.

(d) What are the variables in the study? Identify each as either categorical or quantitative.

(e) What sort of graph would you use to display the results of the study?