Question

One of the assumptions underlying the theory of control charting is that successive plotted points are independent of one another. Each plotted point can signal either that a manufacturing process is operating correctly or that there is some sort of malfunction. Even when a process is running correctly, there is a small probability that a particular point will signal a problem with the process. Suppose that this probability is 0.05. What is the probability that at least one of 10 successive points indicates a problem when in fact the process is operating correctly? What is the probability that at least one of 40 successive points indicates a problem when in fact the process is operating correctly?

Answer 1

a) the probability that at least one of 10 successive points indicates a problem when in fact the process is operating correctly is 0.4013

b) the probability that at least one of 40 successive points indicates a problem when in fact the process is operating correctly is 0.8715

Explanation:

following how the independence multiplication rule works,

i.e finding p(A)' which is 1 - p(No problem) because what we need is an intersection not a union so;

a) 10 successive points

probability (problem) = 0.05

probability (No problem) = 0.95

required probability = 1 - [probability (No problem)]^10

= 1 - (0.95)^10

= 1 - 0.5987

= 0.4013

the probability that at least one of 10 successive points indicates a problem when in fact the process is operating correctly is 0.4013

b) 40 successive points

probability (problem) = 0.05

probability (No problem) = 0.95

required probability = 1 - [probability (No problem)]^40

= 1 - (0.95)^40

= 1 - 0.1285

= 0.8715

the probability that at least one of 40 successive points indicates a problem when in fact the process is operating correctly is 0.8715