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Confidence & Reliability - Reference to a 90/95 Confidence & Reliability Level

Miner

Miner

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#31
#31
Re: Confidence and Reliability - Reference to a 90/95 confidence and reliability leve

Please have patience while waiting for a response. The largest % of users are USA-based and it was 3 AM when you posted.

Reliability is the probability that a product will be functional at specified conditions for a specified amount of time.

Confidence is the amount of uncertainty about this estimate of probability. To increase confidence, you must increase your sample size.
 
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F

Frankli

#32
#32
Re: Confidence and Reliability - Reference to a 90/95 confidence and reliability leve

Thanks for your answer. And, I have another question. How to define the reliability and confidence?
 
F

Frankli

#33
#33
Re: Confidence and Reliability - Reference to a 90/95 confidence and reliability leve

which standard can I get the related information about confidence and reliability?
 
C

Cognizant

#34
#34
Re: Confidence and Reliability - Reference to a 90/95 confidence and reliability leve

N=Log(1-Confidence)/Log(Reliability).... Where N is the required sample size ...
 
D

David-D

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#35
#35
Re: Confidence and Reliability - Reference to a 90/95 confidence and reliability leve

For applications where 0 defects is the desired number then Cognizant's formula is best as it is an exact solution (unlike for one or more failures) and you can use it universally for any one sided, lower confidence bounds. I think my QE team lead gave me the formula and a table of values my first week on the job. I've actually carried a copy in my wallet for years. Alternatively, there's a couple of numbers you can use to get a "close enough" approximation for sample sizes to demonstrate LCLs for reliability.

50% CL = 0.7
80% CL = 1.6
90% CL = 2.3
95% CL = 3.0

So for instance if I wanted to know the numbers of samples to demonstrate 99.5 reliability (1/200) at 90% CL, then multiply 200 times 2.3 to get 460 samples. We find it very useful when looking to prove out automated production and inspection equipment; it might be slightly off of doing the exact calculation but its real close. In those applications, if there are failures/defects then the problem needs to be fixed and requalified so qualifying with zero defects is good.

Thoughts? Thanks,

David
 
T

Todu1

#36
#36
Re: Confidence and Reliability - Reference to a 90/95 confidence and reliability leve

Does anyone have a calculator or spreadsheet for calculating the sample size for 1 (or more) failures?

Much appreciated. Thanks.
 
H

haiping

#37
#37
Re: Confidence and Reliability - Reference to a 90/95 confidence and reliability leve

Miner said:
The only text that I could locate was an abbreviated table on page 6 of this paper.
Click to expand...
Do you know the calculating method of table 3 in this paper?
That is, if we know the sample size is 10, how can we get the knowledge factor p at R=0.99 and C=0.90%?
Thanks.
 
Miner

Miner

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#38
#38
Re: Confidence and Reliability - Reference to a 90/95 confidence and reliability leve

haiping said:
Do you know the calculating method of table 3 in this paper?
That is, if we know the sample size is 10, how can we get the knowledge factor p at R=0.99 and C=0.90%?
Thanks.
Click to expand...
Unfortunately, I do not. You might try the last book cited as a reference. It looked most likely to cover that topic.
 
R

Romvill2002

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#39
#39
Re: Confidence and Reliability - Reference to a 90/95 confidence and reliability leve

Sam Lazzara said:
Here is an excerpt from a document that I prepared that has tables for attribute data confidence and reliability for 0, 1, 2 and 3 failures allowed in the sample, and basic methodology for analyzing variables data to determine upper and lower statistical limits (confidence/reliability analysis).

References are cited below the attribute data tables. They are:
“Reliability Statistics” by R. Dovich (1990; ASQ Press)
F-tables per “Reliability Mathematics” by B.L. Amstadter (1971; McGraw-Hill)
Click to expand...
Hi Sam,

Good table for C n R with C=0,1,2,3. Is it applicable for attributes? Also how do you tie C/R to AQL, LTPD?

R=85% C=90% what would be the corresponding AQL & LTPD for the below sampling plan
n=15, a=0, AQL= ? %, LTPD=? %
n=25, a=1?AQL= ? %, LTPD=? %
What formula should I use to link R, C, AQL & LTPD
 
Last edited:
Sam Lazzara

Sam Lazzara

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#40
#40
Re: Confidence and Reliability - Reference to a 90/95 confidence and reliability leve

Yes, those C/R tables for for attribute (pass/fail) data.
I am not sure how AQL/LTPD can be correlated to C and R.
Perhaps you should start a new thread for that purpose.
 
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