Tricky question

Q. By the rule of seven, a process is said to be out of control if a run of seven samples is found on one side of the process mean. What is the probability that a run of seven occurs on either side of the mean due to random variation?




  • 0.02

  • 0.035

  • 0.0156

  • 0.0273

Explanations:


Answer (c) 1.56% . Correct Answer is 'C'. Let the first sample be on any one side of the mean. The probability that the subsequent sample is on the same side is A, the probability that the third sample is on the same side is again 1/2 A, and so on. The probability for the seventh sample on the same side is also A. Therefore the probability that all the six subsequent points is on the same side as the first is 1/2A * 1/2A * 1/2A * 1/2A * 1/2A * 1/2A = 1/64 = 0.0156 => 1.56%.


 


Can anybody explain why are we not taking 1/2*1/2*1/2*1/2*1/2*1/2*1/2=1/128?


 


 

I did not find a solution myself, but it is quite evident now.

It is like a roulette. 1/2 (random) is a probability to get red as a result of 1 spin. To get the probability of 7 consequent times red you need to multiply the probabilities of every spin 7 times.

Imagine heads and tails game - how easy is to get 7 times tale.

naeem.farooqui's picture

Such type of questions will not appear in actual exam. 

The PMP exam is designed to test one's PM's knowledge rather than Statistical Analysis.

I did not find such question in any Mock as well as in actual exam.

EVM is the most important calculative topic for PMP exam.

This question is from one of the mocktests of simplilearn. The relevance in project management is that this a typ2 error (beta risk) - risk of accepting a false alternate hypothesis which comes under project risk management.

This is a poorly framed Q. You should avoid such Q.

Free Resources For PMP
You will find free resources on almost every website oriented towards PMP. But all free resources are not equal. Many websites provide free stuff just to attract potential customers. The content is dubious and full of errors. You should review the free stuff carefully before using it for your exam prep. I would suggest to all PMP aspirants to look at only reliable sources for doing sample questions. Even a few REPs sometimes provide dubious Q. Unreliable free stuff can lead to failure in the exam.

Free Mock Tests
You should read this article to find reliable sources of 3500+ free questions - PMP Sample Questions.
One should start the exam prep with some reliable free tests but ultimately the final practice should be done on a paid simulator. You should read this article - 13 Reasons to Invest in a Paid PMP Practice Exam.