A pollutioncontrol inspector suspected that a riverside comm

A pollution-control inspector suspected that a riverside community was releasing semi-treated sewage into a river and this, as a consequence, was changing the level of dissolved oxygen of the river. To check this, he drew 15 randomly selected specimens of river water at a location above the town and another 15 specimens below. The sample information and the QQplots by groups are given blow.

1. Use the Wilcoxon Rank-Sum method to test for a difference between the distributions at ?=0.05. Give the p-value and conclusion.

2. Which test (two-sample t or Wilcoxon Rank-Sum test) is appropriate? Justify your response.

Important: Do not use Rcmdr to solve this problem!

mean sd n Above 4.92 0.1567528 15 Below 4.74 0.3202677 15 Above Below o o o norm quantiles norm quantiles

Solution