inclass_nov13 - arises from unavoidable small variations in...

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Stat 296 Fall 2007 The Sign Test Example: In a series of experiments on mengovirus, a microbiologist measured the growth of two strains of the virus—a mutant strain and a nonmutant strain—on mouse cells in petri dishes. Replicate experiments were run on 19 different days. The data are shown below. Each number represents the total growth in 24 hours of the viruses in a single dish. Run 1 2 3 4 5 6 7 8 9 10 11 12 13 14 Nonmutant strain 160 36 82 100 140 73 110 180 62 43 61 14 140 68 Mutant strain 97 55 31 95 80 110 100 100 6 7 15 10 150 44 Signed difference Run 15 16 17 18 19 Nonmutant strain 110 37 95 64 58 Mutant strain 31 14 57 70 45 Signed difference Note that there is considerable variation from one run to another. This variation between runs
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Unformatted text preview: arises from unavoidable small variations in experimental conditions. For instance, both the growth of the viruses and the measurement technique are highly sensitive to environmental conditions such as the temperature and the CO 2 concentration in the incubator. Slight flucuations in the environ-mental conditions cannot be prevented, and these flucuations cause the variation that is reflected in the data. In this kind of situation, the advantage of running the two strains concurrently is particularly striking. From : Statistics for the Life Sciences, by Myra Samuels....
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