Donating Blood to Grandma? Can young blood help old brains? Several studies in mice indicate that it might. In the studies, old mice (equivalent to about a 70-year-old person) were randomly assigned to receive blood plasma either from a young mouse (equivalent to about a 25 -year-old person) or another old mouse. The mice receiving the young blood showed multiple signs of a reversal of brain aging. One of the studies  measured exercise endurance using maximum treadmill runtime in a 90 -minute window. The number of minutes of runtime are given in Table 2.17 for the 17 mice receiving plasma from young mice and the 13 mice receiving plasma from old mice. The data are also available in YoungBlood.

Table 2.17 Number of minutes on a treadmill

Young	27	28	31	35	39	40	45
	46	55	56	59	68	76	90
	90	90	90
Old	19	21	22	25	28	29	29
	31	36	42	50	51	68

(a) Calculate $\bar{x}_{Y}$, the mean number of minutes on the treadmill for those mice receiving young blood.

(b) Calculate $\bar{x}_{O}$, the mean number of minutes on the treadmill for those mice receiving old blood.

(c) To measure the effect size of the young blood, we are interested in the difference in means $\bar{x}_{Y}-\bar{x}_{O}$. What is this difference? Interpret the result in terms of minutes on a treadmill.

(d) Does this data come from an experiment or an observational study?

(e) If the difference is found to be significant, can we conclude that young blood increases exercise endurance in old mice? (Researchers are just beginning to start similar studies on humans.)