Is the water on your airline flight safe to drink? It is not feasible to analyze the water on every flight, so sampling is necessary. In August and September 2004 the EPA found bacterial contamination in water samples from the lavatories and gallery water taps on 20 of 158 randomly selected U.S. flights. Alarmed by the data, the EPA ordered sanitation improvements, and then tested water samples again in November and Decemeber 2004. In the second sample, bacterial contamination was found in 29 of 169 randomly sampled flights. (a) Use a left tailed test at a=.05 to check whether the percent of all flights with contaminated water was lower than the first sample. (b) find the p-value. (c) Discuss the question of significance versus imporance in this specfic application. (d). Test whether normality may be assumed. (Data are from The Wall Street Journal, November 10, 2004 and January 20, 2005.)
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Step by step method for testing the hypothesis under 5 step approach is discussed here. Excel template for each problem is also included. This template can be used to obtain the answers of similar problems.