A random sample of 170 people was provided with a forecasting problem. Each sample member was given, in two ways, the task of forecasting the next value of a retail sales variable. The previous 20 values were presented both as numbers and as points on a graph. Subjects were asked to predict the next value. The absolute forecasting errors were measured. The sample then consisted of 170 differences in absolute forecast errors (numerical minus graphical). The sample mean of these differences was -2.91, and the sample standard deviation was 11.33. Find and interpret the p-value of a test of the null hypothesis that the population mean difference is 0 against the alternative that it is negative. (The alternative can be viewed as the hypothesis that, in the aggregate, people make better forecasts when they use graphs of past history compared to using numerical values from past history.)
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