# Sample Size Formula

Six Sigma – iSixSigma Forums Old Forums General Sample Size Formula

Viewing 20 posts - 1 through 20 (of 20 total)
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• #33930

Fontanilla
Participant

I am trying to determine a random sample for a customer required inspection and the equation that I should use escapes me.  Can anyone assist with a formula that I could use?  Here is the information that I have:
Population of 3000 to choose from
Need 99% accuracy
Need to be 95% confident

What is my sample size?

Thanks

0
#92828

Gabriel
Participant

Are you going to check attributes? If so,
Are you going to count defectives (binomial) or are you going to count defects (Poison)? And what would be the defects or defectives rate you want to detect?
Are you going to measure a variable characteristic? How is it distributed?
And finally, what exactly do you mean by “99% accuracy”? (With an example, please)

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#93066

sanjeev
Member

Hi Dan!
Sample Size is independent of your population size … it depend on  degree of precision of data,  present defect % & Confidence level.
Sample size = (1.96/ Degree of precision)^2 / P *(1-P).
1.96 represents a 95% confidence level.
Regards

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#93073

migs
Participant

Dan,
it seems you are trying to come up with an attribute sampling plan. In addition to Gabriel’s questions:
-How much # of defectsdefectives (or defectdefective rate) you want to accept or reject at what probability?
Rgds

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#93075

Andrews
Participant

What does ‘p’ stand for? If it stands for no. of defects at present , then where is the lot size? Is it not required?

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#93083

Jeff H
Participant

This is the framework for a simple calculation tool I use for doing quality audits in our organization. Sample Size ToolSample Size Calculation Tool(Change)Population Size n 5,000 Enter total population size hereExpected Performance P 0.97 Enter expected rate of performance Level of Precisionp3%Enter level of precision wantedQ 0.03 0.4268- (Note: If you want to use a confidence level other than 95%, 291 you must modify this formula – for 90% confidence use x 124 1.65 instead of 1.96, for 99% use 2.58)Sample Population Size 121 Use thisNote: All sample sizes are based on a 95% confidence level, meaning that the sample characteristic measured by this sample is within the precision range chosen above (typically 3%) 95% of the time.

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#93088

Kesha
Participant

Hello Dan,
The website below has a sample size calculator that you can use to determine your sample size. I hope that this helps :)http://www.surveysystem.com/ssformu.htm

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#93158

vrkp
Member

Dear Jeff,
How did you arrive at such formula ? Can you elaborate with Calculation & the formula. Would be extremely grateful.

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#93183

Jeff
Participant

The sample size calculation tool we use is the required calculation we are required to use in order to determine the accuracy of transactions we process.  It is dictated by our national association.  The formula is:

n =

x

1 + (x – 1)

N

n = sample size required

x = sample size for a proportion calculated from the formula:

x =

1.96

2

(PQ)

p

p = desired precision level (we use 3)

P = expected rate of performance

Q = 100 – P

N = population size

Example:

Population = N = 10,000

Expected performance = P = 95%

Desired precision level = p = 3

Q = 5%

x =

1.96

2

(.95)(.5)

= 0.4268 * 475=203

3

n =

203

=

203

=

203

=

199 items

1 + (203 – 1)

1 + (202/10,000)

1.0202

10,000

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#95985

Aman
Participant

Dear Jeff,
Can u help me on this situation,
A part is not to exceed a failure rate of 3.4 failures in one million we have to find the sample size needed for a hypothesis test of the equality of this failure rate to a 3.4 ppm failure. Risks give is of 0.05 and an uncertainty of +- 10% of the failure rate target

0
#97674

AbetF
Participant

Kesha,
Can you please elaborate more what p ( 0.5 ) and c ( 0.04 ) mean on the site you gave?
abetf

0
#107226

kumar
Participant

Hi all,I have following values and  can someone tell me the formula to calculate the sample size.1. population : 31902. Tolerable error : 10003. expected error : 04. confidence level  : 95 %

0
#108008

Nandkishor Arvind Dhekane
Participant

Our Companies Mobile Audio Technology
Yes.
Software Developer:Nandkishor Arvind Dhekane

0
#108951

Fontanilla
Participant

Jeff,
Do you mind e-mailing me this spreadsheet?
Thanks
[email protected]
Dan

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#187957

asher
Participant

Happy new year ; when we determine sample size the what criterio should be considered(must we have)

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#187959

Darth
Participant

type of data, cost of collecting, power, confidence, precision, variation and most importantly whether there is a full moon or not.

0
#187961

peanut1948
Participant

Cool calculator!  Would you send me a copy of it please?  Thanks!

0
#188824

Boris
Participant

This looks like a great calculator!  Could you please email me a copy, I could definitely use something like this?
Thank you.
Boris

0
#188828

MBBinWI
Participant

I’m sorry, Boris, but it’s not possible to send electronically.  Please tie a red bandana to your post box and I’ll drop off a printed copy on my way home.

0
#188855

Boris
Participant

Thank you, I appreciate that.  No worries though; I thought it was a program or a computer app.  I see the post where you have the equation.  I can use that to create an simple calc. app.
Boris

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