# How Do You Calculate Sample Size?

Six Sigma – iSixSigma Forums General Forums Methodology How Do You Calculate Sample Size?

Viewing 5 posts - 1 through 5 (of 5 total)
• Author
Posts
• #54078

Scott Lawley
Member

Question # 71:

“When sigma is 10, what sample size is needed to specify a 95% confidence interval of +/- 3 units for the mean?

a) 7
b) 11
c) 32
d) 43

I’ve done it a couple of times using what I thought were correct formulae and came up with 30.
I even used different calculators

Thanks,

0
#193422

Straydog
Guest

Are you dealing with proportional or continuous data? What is your confidence that the sigma is 10? It seems like you’re looking for the answer to an exam question and the examiners did not provide essential information, assuming that you’d use an expected formula.

0
#193423

Robert Butler
Participant

What you are looking for it t*s/sqrt(n) = 3 You know s and it looks like the homework problem is assuming t for infinity therefore your answer should be d)

0
#193428

Scott Lawley
Member

@rbutler Thanks, you’re right. I get it now.

0
#193824

sriram
Member

Sample size determination:

(Z*S)^2
n = ——-
E^2
where Z is the confidence level(90%,95%,99%) values (1.645, 1.96, 2.575)
S – Standard deviation,
E = Estimated Mean – True Mean (Xbar – Mu).
Hope this clarifies…

0
Viewing 5 posts - 1 through 5 (of 5 total)

You must be logged in to reply to this topic.