Researchers wondered whether maintaining a patient’s body temperature close to normal by heating the patient during surgery would affect wound infection rates. Patients were assigned at random to two groups: the normothermic group

(patients’ core temperatures were maintained at near normal, 36.5°C, with heating blankets) and the hypothermic group (patients’ core temperatures were allowed to decrease to about 34.5°C).

If keeping patients warm during surgery alters the chance of infection, patients in the two groups should have hospital stays of very different lengths.

Here are summary statistics on hospital stay (in number of days) for the two groups:

Group n x– sx Normothermic 104 12.1 4.4 Hypothermic 96 14.7 6.5

(a) Construct and interpret a 95% confidence interval for the difference in the true mean length of hospital stay for normothermic and hypothermic patients.

(b) Does your interval in part

(a) suggest that keeping patients warm during surgery affects the average length of patients’ hospital stays? Justify your answer.

(c) Interpret the meaning of “95% confidence” in the context of this study.