Reduced heart rate variability (HRV) is known to be a predictor of mortality after a heart attack. One measure of HRV is the average of normal-to-normal beat interval (in milliseconds) for a 24-hour time period.

Twenty-two heart attack patients who were dog owners and 80 heart attack patients who did not own a dog participated in a study of the effect of pet ownership on HRV, resulting in the summary statistics shown in the accompanying table (“Relationship Between Pet Ownership and Heart Rate Variability in Patients with Healed Myocardial Infarcts,” The American Journal of Cardiology [2003]: 718– 721).

Measure of HRV (average normal-to-normal beat interval)

Mean Standard Deviation Owns Dog 873 136 Does Not Own Dog 800 134

a. The authors of this paper used a two-sample t test to test H0: m1 2 m2 5 0 versus Ha: m1 2 m2 0. What assumptions must be reasonable in order for this to be an appropriate method of analysis?

b. The paper indicates that the null hypothesis from Part

(a) was rejected and reports that the P-value is less than .05. Carry out a two-sample t test. Is your conclusion consistent with that of the paper?