Consider the 40 workers presented in Table 3.8 to be a simple random sample from the 1200 workers in the plant.

a. Compute a 90% confidence interval for the population mean forced vital capacity.

b. Suppose it is known, prior to analyzing the data, that the 1200 workers were distributed as follows:

/V, = 1000 (number with high exposure)

N-, = 100 (number with medium exposure)

JV3 = 100 (number with low exposure)

Poststratify the original sample of Table 3.8 and construct a 90% confidence interval for the population mean forced vital capacity.

Compare the intervals of parts

(a) and (b). Which is larger? Why?