SRC Forum - Message Replies
Forum: Reliability & Maintainability Questions and Answers
Topic: Reliability & Maintainability Questions and Answers
Topic Posted by: Reliability & Maintainability Forum
Organization: System Reliability Center
Date Posted: Mon Aug 31 12:47:36 US/Eastern 1998
Posted by: Jeff Jackson
Date posted: Tue Nov 13 22:40:43 US/Eastern 2001
Subject: MIL-STD-471, Test Method 9, 30 Task Minimum????
I have studied MIL-STD-471A, Notice 2, "Maintainability Demonstration," Test Method 9, and want to know if anyone knows why the minimum sample size is 30 maintenance actions. Test Method 9 indicates that the procedure is based on the Central Limit Theorem. Appendix A of the document indicates that the sample of maintenance tasks must be at least four times the number of tasks for the selected test. So, the question is: Is the 30 task minimum somehow determined using the Central Limit Theorem, and if so, how?
Subject: Central Limit Theorem
Reply Posted by: Anthony Coppola
Date Posted: Mon Nov 19 11:06:28 US/Eastern 2001
The central limit theorem states that the mean values of samples taken will follow a normal distribution. However, the fit below 30 samples is not good. Hence, 30 samples is required as a means of assuring that the normal distribution assumption will hold. Below 30 samples, the Students t distribution can be used. This is not as convenient since the Students t is a function of the number of samples, while any normal can be related to the standard normal, so one data table applies to all.