Monthly rates of return on the shares of a particular common stock are independent of one another and normally distributed with a standard deviation of 1.8. A sample of 15 months is taken. a) Find the probability that the sample standard deviation is less than 2.6. b) Find the probability that the sample standard deviation is more than 1.2.

Anew test for ra is being developed. the test is administered to 750 patients with ra and 750 without ra. for the sake of the question, assume the prevalence of ra in this population is 50%. of the 750 patients with ra, 575 have a positive result. of the 750 patients without ra, 150 have a positive result. what is the positive predictive value of the new test? a575/(575+150) b. 600/(600+ 150) c. 575/(575+125) d. 600/(600+ 125)