nm

60 where Z has a standard normal distribution and the resulting probability is obtained from Appendix Table 1 using the procedures developed in Chapter 5. From this analysis we conclude that the probability that the sample mean will be greater than 14.4% is only 0.0336. If a sample mean greater than 14.4% actually oc- curred, we might begin to suspect that the population mean is greater than 12.2%. or that we do not have a random sample that properly represents the population prob- ability distribution Example 6.3 Spark Plug Life (Normal Probability) A spark plug manufacturer claims that the lives of its plugs are normally distributed with a mean of 60,000 miles and a standard deviation of 4,000 miles. A random sample of 16 plugs had an average life of 58,500 miles. If the manufacturer's claim is correct. what is the probability of finding a sample mean of 58,500 or less