# How Do You Calculate Sample Size?

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

- This topic has 4 replies, 3 voices, and was last updated 8 years ago by sriram.

- AuthorPosts
- May 27, 2012 at 6:09 pm #54078

Scott LawleyMember@slawley**Include @slawley in your post and this person will**

be notified via email.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) 43I’ve done it a couple of times using what I thought were correct formulae and came up with 30.

I even used different calculatorsThanks,

0May 27, 2012 at 11:46 pm #193422

StraydogGuest@Straydog**Include @Straydog in your post and this person will**

be notified via email.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.

0May 28, 2012 at 5:21 am #193423

Robert ButlerParticipant@rbutler**Include @rbutler in your post and this person will**

be notified via email.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)

0May 28, 2012 at 10:09 am #193428

Scott LawleyMember@slawley**Include @slawley in your post and this person will**

be notified via email.@rbutler Thanks, you’re right. I get it now.

0July 25, 2012 at 4:06 am #193824

sriramMember@sriram54321**Include @sriram54321 in your post and this person will**

be notified via email.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 - AuthorPosts

You must be logged in to reply to this topic.