In a random sample of patient records in Cutter Memorial Hospital, six-month postoperative exams were given in 90 out of 200 prostatectomy patients, while in Paymor Hospital such exams were given in 110 out of 200 cases. In comparing these two proportions, normality may be assumed because
n pi > pr equal to 10 and n(1 -pi) 10 for each sample taken separately.
the samples are random, so the proportions are unbiased estimates.
the probability of success can reasonably be assumed constant.
the populations are large enough to be assumed normal.