(a) Calculate a 95% con dence interval for the average concentration of lead in the area, using the SRS sample. Repeat the same for the average concentration of copper; (b) Now use the poststrati ed sample, and nd 95% con dence intervals for the average concen- tration of lead and copper. How do these compare with the con dence intervals obtained in Part (a)? Do you think that using strati cation in future surveys would increase prediction