Spam mail percentages. The National Technology Readiness Survey sponsored by the Smith School of Business at the University of Maryland surveyed 418 randomly sampled Americans, asking them how often they delete spam emails. In 2004, 23% of the respondents said they delete their spam mail once a month or less, and in 2009 this value was 16%.
(a) What are the hypotheses for evaluating if the proportion of those who delete their email once a month or less has changed from 2004 to 2009?
(b) What is the point estimate for the difference between the two population proportions?
(c) A report on the survey states that the observed decrease from 2004 to 2009 is statistically significant. Explain what this means in context of the hypothesis test and the data.
(d) Would you expect a confidence interval for the difference between the two population proportions to contain 0? Explain your reasoning.