Sampling plans

DEVELOPING A SAMPLING PLAN Step 1 Sample Size Selection 

Sample size selection can either be statistically based or arbitrarily chosen. Arbitrary sampling plans are plans in which the number of samples is arbitrarily set at some 

Determining the sample size is often a difficult decision, and sometimes has to be made without sufficient information. If there is uncertainty, sometimes a quick small informal test of the system may help decide what is the best way to proceed. 

To avoid bias and other sampling errors, the containers to be sampled must be randomly selected. To make a random selection, first number all containers in the lot, and then use a random number table (or computer generated set of random numbers) to choose from which container or containers to withdraw the samples. See Appendix I for an example on the use of random number tables. 

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Five questions should be considered when designing a sampling plan ... 

Example of a systematic sampling plan for collecting samples from a lake. Each solid dot represents a sample collected from within the sampling grid. 

A sampling plan that combines judgmental sampling with systematic sampling. 

A sampling plan that divides the population into distinct strata from which random samples are collected. 

A sampling plan in which samples are collected because they are easily obtained. 

An analysis requires a sample, and how we acquire the sample is critical. To be useful, the samples we collect must accurately represent their target population. Just as important, our sampling plan must provide a sufficient number of samples of appropriate size so that the variance due to sampling does not limit the precision of our analysis. 

More attention to selecting and obtaining a representative sample. The design of a statistically based sampling plan and its implementation are discussed earlier, and in more detail than in other textbooks. Topics that are covered include how to obtain a representative sample, how much sample to collect, how many samples to collect, how to minimize the overall variance for an analytical method, tools for collecting samples, and sample preservation. 

Sampling Plan. The first step to assuring accuracy, ie, the conformity of the measured value to the tme or expected value (23), is to obtain a representative sample. Sampling may be the responsibiUty of either production or of QC. A written sampling plan approved by QC should be followed. It is the responsibihty of the QC unit to ensure that samples represent all the material under evaluation. 

A thorough sampling plan should describe when the sample is to be taken and how many samples are required. It also specifies from what location within the equipment, such as the manway or discharge valve, to take the sample. The plan should also indicate what sampling equipment and sample container should be used, as well as the type of tests to be performed and the acceptance criteria. 

The standard requires the acceptance criteria for attibute data sampling plans to be zero defects. 

Don t derive your sampling plans from unproven statistical methods. 

In designing a sampling plan the following points should be considered 8... 

When using the defect-detection approach to quality control certain problems develop. Inspection does nothing to improve the process and is not very good at sorting good-from-bad. Also, sampling plans developed to support an acceptable quality level (AQL) of 5%, for example, say that a company is content to deliver or reject 5% defects. 

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