28. Heart Rate Variability. A reduction in heart rate variability is associated with elevated levels of stress, because the body continues to pump adrenaline after high-stress situations, even when at rest.20 Suppose the standard deviation of heartbeats in the general population is 20 beats per minute, and that the population of heart rates is normal. A random sample of 50 individuals leading high-stress lives has a sample variance of 200 beats per minute. Test, using level of significance , whether the population standard deviation for those leading high-stress lives is lower than that in the general population.