Today we will talk about the variability of the sample average. This leads to an interesting computation: how many people do you need to interview in order to ensure that a confidence interval for the population mean has a given width (or smaller). We'll have a quiz next class. To prepare for the quiz we recommend that you review: (1) differences between mean and median and how to compute them; (2) usage of the phrases "sample average", "population average", "parameter", "statistic"; (3) how to compute a confidence interval using bootstrapping; (4) how to define and compute the standard units for a value in an array; (5) properties and examples of bell-shaped distributions; (6) the use of p-values in hypothesis tests; (7) when to reject the null in a hypothesis test and what that means.