In a test of corrosion resistance, a sample of 60 Incoloy steel specimens were immersed in acidified brine for 4 hours, after which each specimen had developed a number of corrosive pits. The maximum pit depth was measured for each specimen. The mean depth was 850 μm with a standard deviation of 153 μm. The specification is that the population mean depth μ is less than 900 μm.
a. Find the P-value for testing H0: μ ≥ 900 versus H1: μ < 900.
b. Do you believe it is plausible that the mean depth is at least 900µm, or are you convinced that it is less than 900µm? Explain.