# Sample Size Calculation

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Viewing 6 posts - 1 through 6 (of 6 total)
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• #54753

Sahab
Participant

Hi
I am looking on how to do a sample size calculation on attribute data

Population: 1000 tests per month
Pass Rate: 85% of tests are correct

How can I determine of the 1000 tests per month, how many tests i would need to do at 95% CI based on 85% of the tests being correct every month. I don’t want to test all 1000 every month, just a sample of them

also how can I do this calculation in Minitab?

thanks

0
#196989

Katie Barry
Participant

@singhsahab — Consider describing how you would approach this problem. What you’ve tried, where you’re getting stuck, what specifically you don’t understand. iSixSigma readers are likely to help but they’re not here to do your work for you.

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#196990

Prabhu V
Participant

Hi,

For discrete data

N=(Z/E)^2*p(1-p)

Z – Constant for confidence level (like 1.645  90%, 1.96  95%, 2.575  99%)
E  Precision or Error  level of precision desired from the sample in units of proportion (say 5% or 0.05, if more critical then use 1% or 0.01)
p- Proportion(like 15% or 0.15 defect rate in your case)
Please feel free to post your queries in case if you want for clarifications.

Thanks and regards
Prabhu V.

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#196991

Chris Seider
Participant

Stat>Power and Sample Size>1 proportion

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#197004

shubin
Guest

If 95% confidence (two sided) @ 85% proportion with 5% margin of error, some resuls for your reference.
1. Per Prabhuv’s formula (Central limit therom), Sample Sze=195
2. Minitab (Stat-Sample size & power-Sample size for Estimation), sample size=254.
If 95% confidence (one sided, say upper bound), Sample size=124.

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#197079

Russell
Participant

Many great free calculators on the internet too…..

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