chapter 1

Chapter 9: Inferences Based on Two Samples66.a.1400Comparative Boxplot of Tree Density BetweenFertilizer Plots and Control Plots1300Fertiliz120011001000Fe rtilizCo ntrolAlthough the median of the fertilizer plot is higher than that of the control plots, thefertilizer plot data appears negatively skewed, while the opposite is true for the controlplot data.b. A test of µ − µ 0H vs. H µ − µ 00:1 2=a:1 2≠yields a t value of -.20, and a twotailedp-value of .85. (d.f. = 13). We would fail to reject H o ; the data does not indicate asignificant difference in the means.c. With 95% confidence we can say that the true average difference between the tree densityof the fertilizer plots and that of the control plots is somewhere between –144 and 120.Since this interval contains 0, 0 is a plausible value for the difference, which furthersupports the conclusion based on the p-value.67. Let p 1 = true proportion of returned questionnaires that included no incentive; p 2 = trueproportion of returned questionnaires that included an incentive. The hypotheses areH p − p 0 vs. H p − p 00:1 2=0:1 21pˆnumerator of the z statistic will be > 0, and since we have a lower tailed test, the p-value willbe > .5. We fail to reject H o . This data does not suggest that including an incentive increasesthe likelihood of a response.284