25. The objective of a study by Martin et al. (A-28) was to compare the function of neutrophils in the pulmonary artery blood and lung lavage fluid of patients early in the course of adult respiratory distress syndrome. Of concern were three antibacterial functions: the release of reactive oxygen species, microbiocidal activity for a target organism, Staphylococcus aureus, and chemotaxis. For 18 of the subjects in the study the mean bronchoalveolar lavage fluid pH was 7.39 with a standard deviation of .39.

Construct a 90 percent confidence interval for the population mean pH. State the assumptions necessary to make your procedure valid.