A pollution-control inspector suspected that a riverside community was releasing semitre...

A pollution-control inspector suspected that a riverside community was releasing semitreated 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 dissolved oxygen readings, in parts per million, are given in the accompanying table.

Use the computer output shown here to answer the following questions.

Boxplots of above- and below-town specimens (means are indicated by solid circles)

a. Do the data provide sufficient evidence to indicate a difference in mean oxygen content between locations above and below the town? Use α = .05.

b. Was the pooled t test or the separate-variance t test used in the computer output?

c. Do the required conditions to use the test in (a) appear to be valid for this study? Justify your answer.

d. How large is the difference between the mean oxygen content above and below the town?