When making the calcualtions, you have to be careful jsut what you mean to find.
- Do you want to be fairly certain to reject a lot that is at least 0.5% defective (and perhaps also reject some lots that are less defective)?
- Do you want to be fairly certain to accept a lot that is at most 0.5% defective (and perhaps also accept some lots that are more defective)?
- Do you want to know the actual defect rate within some range (for example "I am 95% certain the defect rate is 0.5% +/- 0.1%
Each of these would require a different approach.
A while ago a posted a spreadsheet that helps you determine an appropriate sampling plan based on specified defect rates. It is described in this thread:
http://elsmar.com/Forums/showthread.php?t=12836
The spread sheet is set up to enter 4 numbers. For example 5% chance of accepting a lot with 0.5% defective and 5% chance of rejecting a lot with 0.1% defective. Then you play with the sample size until you find something that works. For such small defect rates, you typically need very large numbers -- for the example I just gave, you would draw about 1300 parts - accepting 2 defects and rejecting 3 defects.\
Tim F