The Elsmar Cove Forum Validate 99.5% reliability 90% confidence - Sample size calculation
 Forum User Name Keep Me Logged In Password
 Register Photo Albums Blogs FAQ Registered Visitors Social Groups Calendar Search Today's Posts Mark Forums Read

 Elsmar Cove Forum Visitor Notice(s) The Christian holiday of Good Friday is tomorrow and Easter is Sunday. Many people are on holiday so expect the forum to be slow for about a week.

 Search the Elsmar Cove @import url(http://www.google.com/cse/api/branding.css); Custom Search Monitor the Elsmar Forum Monitor New Forum Posts Follow Marc & Elsmar Elsmar Cove Groups Sponsor Links Donate and \$ Contributor Forum Access Courtesy Quick Links Links that Elsmar Cove visitors will find useful in your quest for knowledge: Howard'sInternational Quality Services Marcelo Antunes'SQR Consulting Bob Doering'sCorrect SPC - Precision Machining NIST's Engineering Statistics Handbook IRCA - International Register of Certified Auditors SAE - Society of Automotive Engineers Quality Digest Portal IEST - Institute of Environmental Sciences and Technology ASQ - American Society for Quality

#1
5th December 2008, 05:39 PM
 doops Involved in Discussions   Registration Date: Jul 2008 Posts: 10 Thanks Given to Others: 5 Thanked 0 Times in 0 Posts Karma Power: 25 Karma: 10
Validate 99.5% reliability 90% confidence - Sample size calculation

If I state a component is 99.5% reliable at 90% confidence, I can calculate the sample size needed to validate the R&C target is 460 samples. I would expect to see .5% cumulative failures in the field for the life of the product. Sample size = ln(1-C)/ln(R)

Someone has stated if the test is performed using the 95th percentile usage profile (more harsh usage profile than the median user), then I can validate the same reliability using a smaller sample size. For example, I can use a sample size of 22, to meet 99.5% reliability with 90% confidence if I use a 95th percentile usage profile. I expect .5% cumulative failures in the field for the life of the product?

Are the previous statements correct? Can I reduce the sample size from 460 to 22, and validate 99.5% reliability of the population by adjusting the usage profile?

Calculation:
I test to R90/C90, I need 22 samples.
Population Reliability = 1-(1-95th percentile user)(1-Reliability)
=1-(1-.95)(1-.9)
=R99.5%

#2
8th December 2008, 06:14 AM
 Coury Ferguson Moderator here to help   Registration Date: Jan 2006 Location: SoCal Age: 56 Posts: 4,321 Thanks Given to Others: 941 Thanked 621 Times in 510 Posts Karma Power: 477 Karma: 5825
Re: Validate 99.5% reliability 90% confidence - Sample size calculation

Can someone help here?

__________________

"The one thing I want to leave my children is an honorable name." "It is hard to fail, but it is worse never to have tried to succeed." Theodore Roosevelt Chicago, IL, April 10, 1899
 Thanks to Coury Ferguson for your informative Post and/or Attachment!
#3
12th December 2008, 05:03 PM
 w_grunfeld Willy Grunfeld   Registration Date: Jul 2005 Location: Israel Posts: 228 Thanks Given to Others: 19 Thanked 65 Times in 51 Posts Karma Power: 59 Karma: 868
Re: Validate 99.5% reliability 90% confidence - Sample size calculation

Someone has stated if the test is performed using the 95th percentile usage profile (more harsh usage profile than the median user), then I can validate the same reliability using a smaller sample size.

I never heard of 95th percentile usage profile and don't know what is meant by that. It doesn't sound right to me. The word harsher sugests you are referring to some accelaration factor. Other than temperature which can be used as an acceleration factor and for which the relationships are known I don't know of any acceleration factor that can be used readily.
Assuming that you are referring to accelerated testing, the test time can be reduced or conversely the sample size if the test time remains unchanged. I wouldn't take that formula as correct unless you have the full reference how is it derived. ooks suspicious to me

__________________

Willy Grunfeld
RAMSQ Consultant
 Thanks to w_grunfeld for your informative Post and/or Attachment!
#4
12th December 2008, 05:18 PM
 w_grunfeld Willy Grunfeld   Registration Date: Jul 2005 Location: Israel Posts: 228 Thanks Given to Others: 19 Thanked 65 Times in 51 Posts Karma Power: 59 Karma: 868
Re: Validate 99.5% reliability 90% confidence - Sample size calculation

"If I state a component is 99.5% reliable at 90% confidence, I can calculate the sample size needed to validate the R&C target is 460 samples. I would expect to see .5% cumulative failures in the field for the life of the product. Sample size = ln(1-C)/ln(R)"

99.5% reliable means there is a 0.995 probability it will fail at time t. You can't "expect" =calculate the number of failures without knowing the failure rate and the time.
90% confidence is just what it sounds- that there is a 90 %probability that the actual reliability will indeed be 99.5% and a 10% chance that it won't.
I don't know where did you take that formula to calculate the sample size....you can't talk about sample size without taking time into consideration. The assumption (for constant failure rate) is that 100 parts operating for 1,000 hours is the same as 1000 parts operating for 100 hours.

__________________

Willy Grunfeld
RAMSQ Consultant
 Thanks to w_grunfeld for your informative Post and/or Attachment!
#5
23rd December 2008, 10:53 AM
 doops Involved in Discussions   Registration Date: Jul 2008 Posts: 10 Thanks Given to Others: 5 Thanked 0 Times in 0 Posts Karma Power: 25 Karma: 10
Re: Validate 99.5% reliability 90% confidence - Sample size calculation

The sample size equation comes from the number of samples for the lower binomial confidence bound, for a zero failure test. The R&C are life targets, and the zero failure test is for 1 life. I should have clarified.

 The Elsmar Cove Forum Validate 99.5% reliability 90% confidence - Sample size calculation

 Bookmarks

 Visitors Currently Viewing this Thread: 1 (0 Registered Visitors (Members) and 1 Unregistered Guest Visitors)

 Forum Posting Settings You may not post new threads You may not post replies You may not post attachments You may not edit your posts BB code is On Smilies are On [IMG] code is On HTML code is Off Forum Rules

 Similar Discussion Threads Discussion Thread Title Thread Starter Forum Replies Last Post or Poll Vote RMedrano Inspection, Prints (Drawings), Testing, Sampling and Related Topics 8 17th March 2008 08:01 PM Nissim Shaked Inspection, Prints (Drawings), Testing, Sampling and Related Topics 1 19th September 2006 10:04 AM rexlee Inspection, Prints (Drawings), Testing, Sampling and Related Topics 5 14th November 2005 10:57 AM Benoit Quality Tools, Improvement and Analysis 5 21st January 2005 09:38 AM Mark Smith Statistical Analysis Tools, Techniques and SPC 3 22nd July 1999 10:52 AM

The time now is 08:27 PM. All times are GMT -4.