In a randomized comparative experiment on the effect of calcium in the diet on blood pressure, researchers divided 54 healthy white males at random into two groups. One group received calcium; the other, a placebo. At the beginning of the study, the researchers measured many variables on the subjects. The paper reporting the study gives x = 114.9 and s = 9.3 for the seated systolic blood pressure of the 27 members of the placebo group. 

1. Give a 95% confidence interval for the mean blood pressure in the population from which the subjects were recruited.

2. What conditions for the population and the study design are required by the procedure you used in (a)? Which of these conditions are important for the validity of the procedure in this case?