Spam mail counts. The 2004 National Technology Readiness Survey sponsored by the Smith School of Business at the University of Maryland surveyed 418 randomly sampled Americans, asking them how many spam emails they receive per day. The survey was repeated on a new random sample of 499 Americans in 2009.44
(a) What are the hypotheses for evaluating if the average spam emails per day has changed from 2004 to 2009.
(b) In 2004 the mean was 18.5 spam emails per day, and in 2009 this value was 14.9 emails per day. What is the point estimate for the difference between the two population means?
(c) A report on the survey states that the observed difference between the sample means is not statistically significant. Explain what this means in context of the hypothesis test and data.
(d) Would you expect a confidence interval for the difference between the two population means to contain 0? Explain your reasoning.