USA Today reports (Darryl Haralson, “It’s All about Overstock.com”) on an ad for Overstock.com, which sells discounted merchandise on its Web site. To evaluate the effectiveness of the ads, Harris Interactive conducted a nationwide poll of 883 adults. Of the 883 adults, 168 thought the ads were very effective. This was compared to the Harris Ad Track average of 21%.
a. Determine if the sample size is large enough for the test to warrant approximating the sample proportion’s distribution with a normal distribution.
b. Does the Harris poll provide evidence to contend that the proportion of adults who find Overstock.com’s ads to be very effective is smaller than the Harris Ad Track average? Use a significance level of 0.05.