What is the estimate of the standard deviation of the sampling distribution of sample proportion for whenever?

[wholegrain]

A Quality Analyst wants to construct a control chart for determining whether four machines, all producing the same product, are under control with regard to a particular quality attribute. Accordingly, she inspected 1,000 units of output from each machine in random samples, with the following results:

23/1000
15/1000
29/1000
13/1000

What is the estimate of the standard deviation of the sampling distribution of sample proportion for whenever this process is under control?

 

I used the formula for standard deviation for population samples and it didn't give me the right answer.

 

I think the answer was 0.0044. I have no idea how they got there.

Edited October 27, 2013 by wholegrain

[daniton]

Show us what you have done and let us help you.

[wholegrain]

I took the average which is 20 and took the square root of that value (p-chart = poisson distribution?)and then divided it by square root of 1000, which didn't give me the correct answer.