Determining correct sample size
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 This topic has 15 replies, 12 voices, and was last updated 20 years ago by vidya Kulkarni.

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April 19, 2002 at 9:44 pm #29284
Miguel CamargoParticipant@MiguelCamargo Include @MiguelCamargo in your post and this person will
be notified via email.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,
0April 23, 2002 at 12:13 pm #74673Hi 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
0April 23, 2002 at 12:44 pm #74675Could you be so kind as to send me the Excel file as well? It would be very helpful…
My email address is: [email protected]
Thanks much!
Corina0April 23, 2002 at 12:48 pm #74676Assuming 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
If your data is binomial
n = Z^2*p*(1p)/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 detect0April 23, 2002 at 1:18 pm #74677
john beaudoinParticipant@johnbeaudoin Include @johnbeaudoin in your post and this person will
be notified via email.My email is [email protected]
Could you please forward me a copy as well?0April 23, 2002 at 1:22 pm #74678
john beaudoinParticipant@johnbeaudoin Include @johnbeaudoin in your post and this person will
be notified via email.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.
0April 23, 2002 at 3:13 pm #74687Prostats for palm has sample size calculator. In addition I use an excel worksheet to calculate sample size.
0April 23, 2002 at 10:19 pm #74739
Miguel CamargoParticipant@MiguelCamargo Include @MiguelCamargo in your post and this person will
be notified via email.Hi,Here is my email address:[email protected]
0April 24, 2002 at 4:13 pm #74769Please send me a copy
my email [email protected]
thanks
JJS0April 24, 2002 at 4:22 pm #74770
Ted J. LoeffelholzMember@TedJ.Loeffelholz Include @TedJ.Loeffelholz in your post and this person will
be notified via email.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.
0April 24, 2002 at 7:43 pm #74786
CAconsultantParticipant@CAconsultant Include @CAconsultant in your post and this person will
be notified via email.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 Xbar R control charts to sample across days, shifts, operators, etc.
Hope this helps.
Doug0April 24, 2002 at 7:53 pm #74788This is a popular excel file! Could I get a copy?
[email protected]
Thanks!0August 14, 2002 at 2:14 am #78074Hi, Sridhar. I’ll appreciate if you can send me a copy. Email [email protected]. Thanks & Regards
James0August 14, 2002 at 9:37 am #78076
A. JerezParticipant@A.Jerez Include @A.Jerez in your post and this person will
be notified via email.My email 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<<0August 14, 2002 at 9:39 am #78077
A. JerezParticipant@A.Jerez Include @A.Jerez in your post and this person will
be notified via email.Kindly send me a copy of your excel file as well…
EMAIL ADD: [email protected]..
Thank You and More power!0August 14, 2002 at 11:02 am #78079
vidya KulkarniMember@vidyaKulkarni Include @vidyaKulkarni in your post and this person will
be notified via email.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]
vidya0 
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