# Determining correct sample size

Six Sigma – iSixSigma Forums Old Forums General Determining correct sample size

Viewing 16 posts - 1 through 16 (of 16 total)
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• #29284

Miguel Camargo
Participant

Hi all,We want to know what percentage of the rejects are caused because of assembly issues (the failures because of assembly have the same symptoms than the ones failing because of some other reason). However, we need to perform a failure analysis that consumes a lot of time. So that, we would like to analyze the rejects for several lots as a way to get a picture of the process. Take in mind that there is a lot variation in the process, so that we need to analyze as much lots as we can & we would like the sample to be representative for all the working shifts.If there any statistical methodology for calculating this?Any comment is welcome,

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#74673

Sridhar
Member

Hi Camargo,
Sample size can be determined based on the following three factros
1. size of population
2. margin of error
3. probability of failure or sucess
There is a simple formula to calculate the sample size. Send me your mail id i will send you one excel sheet which calculates the sample size if you give the paratmeters whatever required.
thanks
A.Sridhar

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#74675

Corina
Participant

Could you be so kind as to send me the Excel file as well? It would be very helpful…
My e-mail address is: [email protected]
Thanks much!
Corina

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#74676

abasu
Participant

Assuming you have a normal distribution you can estimate an appropriate sample size using the formula below
n = Z^2*s^2/d^2 where
Z = sigma level of the confidence (1.96 for 95% confidence level)
s = standard deviation of your process and
d = the difference you want to detect
n = Z^2*p*(1-p)/d^2 where
Z = sigma level of confidence (1.96 for 95% confidence)
p = proportion or failure rate and
d = the difference you want to detect

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#74677

john beaudoin
Participant

My e-mail is [email protected]
Could you please forward me a copy as well?

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#74678

john beaudoin
Participant

You are exactly right, and have also shown why sample size hangs up a lot of individuals.  There is no real simple way to determine sample size.  However, I have heard that there are software solutions to such.  Are you doing these calculations by hand (calculator) or on a spreadsheet?  Do you know of any software such as Excel addins or PDA (Palm Pilot) software, etc. that would aid in this.  We picked up a lot of documentation on ANSI standards with a slide rule type calculator and some other tables and things, but wanted something a little more solid.

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#74687

abasu
Participant

Prostats for palm has sample size calculator.  In addition I use an excel worksheet to calculate sample size.

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#74739

Miguel Camargo
Participant

Hi,Here is my e-mail address:[email protected]

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#74769

JJS
Participant

my email [email protected]

thanks

JJS

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#74770

Ted J. Loeffelholz
Member

Be careful with your sample selection.  I noticed that you mentioned lots and shifts.  Do you know for sure that your failures are not lot or shift dependent?  If you don’t then you will probably want to make sure you know if your manufacturing process is a fully nested process and pick your samples accordingly.  This would give you more information as to where in your process the variation is occurring.

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#74786

CAconsultant
Participant

A simple formula to use would be n x p >5, where n = sample size and p=current proportion defective.
First calculate your current % defective. (total rejects/total produced).  Plug that number in for p and solve for n.
Once to have that you can develop subgrouping strategies and use X-bar R control charts to sample across days, shifts, operators, etc.
Hope this helps.
Doug

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#74788

DC
Participant

This is a popular excel file!  Could I get a copy?
[email protected]

Thanks!

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#78074

Saherngu
Participant

Hi, Sridhar. I’ll appreciate if you can send me a copy. Email [email protected]. Thanks & Regards
James

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#78076

A. Jerez
Participant

My e-mail add is [email protected] yahoo.com.
I am interested with your excel file. Kindly send me a copy as well..
Thank You and More Power!
>>TOXICO<<

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#78077

A. Jerez
Participant

Kindly send me a copy of your excel file as well…
Thank You and More power!

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#78079

vidya Kulkarni
Member

Hi Sridhar,
Iam also interested to know how to determine the correct sample size.
Pl. mail me excel sheet too.
my mail ID is [email protected]

vidya

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