Number of samples needed? 1

[Dryver]

I need to find out a good way to estimate the number of samples I need to take for an analysis. I will have some HLW and LAW simulant that I will have tested for certain markers. I need to make sure that there is only a +/- 1% difference from the control specimen to my samples. Statistically how do I determine how many samples I need to take to get a accurate result?

Thanks a lot,

 

[jmw]

Did you do a search on the interent for least squares or Chi?



JMW

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[m777182]

It will not be so easy!First you have to decide what proportion of bad units is acceptable for you but do not say zero, because the lower the proportion of unconforming units that seems acceptable to you the higher is the cost of controlling the quality. The concept of "zero fault" will ask for controlling each unit and that could be quite a problem- imagine that you produce 100 units per second that you want to controll by three or more atributes!
The decision about the accepted quality level is thus the very crucial for your sampling plans and depends on your company quality policy.
The procedure to determine the number of sample units is described in ISO 2859-1,2 and 3(1989)and ISO 3951. Regrettfully I do not know the corresponding ASTM standard but nevertheless, numbers in tables are the same because they are calculated on the same statistics.For the concept of the AQL (ISO 2859) you decide about accepted quality level, you enter the numbers of units in a lot and you choose the sampling plan: you take samples once, twice or more times. With those data you find the number of units you have to controll.
The entire proceedure is available with some commercial software.Personally I like "Statistica" or at the price top end "SAS", but I am sure you can find many more.But with a standard in a paper print form you can do it as well.
If you want to controll all units, there is an ellegant way to employ video camera (or more cameras at the same time) and an image analysing system.I have seen a system with five cameras operating parallely in a real time recognizing among a few thousand objects newly defined in MS Windows.
But as I said, your company's quallity policy will define the number of sample units in a lot that should be contolled.
m777182