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62 CHAPTER 4 Did We Meet or Exceed Our Goal? To run the statistical test using a confidence interval, set the confidence to 80% for a one-sided alpha of 0.1. The critical value of t for 80% confidence and 5 degrees of freedom is 1.476. The stan- dard error of the mean, as shown in the previous equation, is 0.111. The critical difference for the confidence interval, therefore, is 1.476(0.111), or about 0.164, so the confidence interval of the log values ranges from 0.861 to 1.189. Using the EXP function to convert these natural log values back to times in minutes, the confidence interval ranges from 2.4 to 3.3 minutes. The upper bound of the confidence limit exceeds the criterion of 3 minutes, so the results do not support the claim that most callers would complete the task in less than 3 minutes. The confidence interval does suggest that, given the data in hand, most callers would complete the task in less than 3.5 minutes. References Agresti, A., Franklin, C.A., 2007. Statistics: The Art and Science of Learning from Data. Pearson, New York. Armitage, P., Berry, G., Matthews, J.N.S., 2002. Statistical Methods in Medical Research, 4th ed. Blackwell Science, Oxford, UK. Bangor, A., Kortum, P.T., Miller, J.T., 2008. An empirical evaluation of the system usability scale. Int. J. Hum. Comput. Interact. 6, 574­594. Chapanis, A., 1988. Some generalizations about generalization. Hum. Fact. 30, 253­267.