Historically,the average proportion of defective bars has been 0.015.Samples will be of 100 bars each.Construct a p-chart using z = 3.Suppose a sample had 0.07 defectives.What would you do?
A)Nothing;it is just random variation.
B)Look for assignable causes.
C)Change z to 2 and take another sample.
D)Change z to 4 and continue sampling.

Question

Quizplus · Accepted Answer

A p-chart is used to monitor the proportion of defective items in a process. In this case, the historical proportion of defective bars is 0.015, and the sample size is 100. By using z=3, we are setting our control limits at +-3 standard deviations from the mean proportion of defective bars. 
If a sample had 0.07 defectives, this would be significantly outside of our control limits (which would be at approximately 0.015+-0.024). This suggests that there may be an assignable cause impacting the proportion of defective bars. Therefore, we would need to investigate and look for assignable causes before deciding on any further action. Changing the z value or taking another sample would not address the underlying issue of an assignable cause.

Historically,the Average Proportion of Defective Bars Has Been 0