FromStatisticsandDataAnalysisfromElementarytoIntermediatebyTamhaneand Dunlop, pg 265. Diabetic patients monitor their blood sugar levels with a home glucose monitor which analyzes a drop of blood from a finger stick. Although the monitor gives precise results in a laboratory, the results are too variable when it is used by patients. A new monitor is developed to improve the precision of the assay results under home use. Home testing on the new monitor is done by 25 persons using drops from a sample having a glucose concentration of 118 mg/dl. If < 10 mg/dl, then the precision of the new device under home use is better than the current monitor. The reading from 25 tests are as follows:

125 123 117 123 115 112 128 118 124 111 116 109 125 120 113 123 112 118 121 118 122 115 105 118 131

a) Make a normal probability plot of these readings. Is it reasonable to assume that the data follow a normal distribution? Justify your answer.

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b) Determine a 90% confidence interval for the population standard deviation. Can we say that the precision of the new device under home use is better than the current monitor? Justify your answer.