The reliability of wafer level- chip-scale packages mounted on a printed circuit board (PCB) in handheld devices such as cellular phones, pagers, and PDAs was investigated by researchers at National Semiconductor Corporation (Electronic Components and Technology Conference, May 2005). In one experiment, PCBs were mounted 2 millimeters from the point of plunger contact and the number of cycles to failure measured for each. The data (simulated) for a sample of 20 PCBs are listed in the table. Assume the number of cycles to failure can be approximated by an exponential distribution.

a. Find and interpret a 95% confidence interval for the mean number of cycles to failure.

b. Find and interpret a 95% confidence interval for the hazard rate of the distribution.

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PCB3 1534 333 1179 679 1186 197 279 263 682 240 331 508 361 593 420 2028 271 176 525 538