In restaurants, servers rely on tips as a major source of income. Does serving candy after the meal produce larger tips? To find out, two waiters determined randomly whether or not to give candy to 92 dining parties. They recorded the sizes of the tips and reported that guests getting candy tipped an average of 17.8% of the bill, compared with an average tip of only 15.1% from those who got no candy. ( "Sweetening the Till: The Use of Candy to Increase Restaurant Tipping." Journal of Applied Social Psychology 32, no. 2 [2002]: 300 309)
a) Was this an experiment or an observational study? Explain.
b) Is it reasonable to conclude that the candy caused guests to tip more? Explain.
c) The researchers said the difference was statistically significant. Explain in this context what that means.