What is Sampling?
Sampling is the selection of a set of elements from a target population or product lot. Sampling is frequently used because gathering data on every member of a target population or every product produced by a company is often impossible, impractical, or too costly to collect. Sampling lets you draw conclusions or make inferences about the population or product lot from which the sample is drawn (Figure 1).
Avoiding Sampling Errors
When used in conjunction with randomization, samples provide virtually identical characteristics relative to those of the population or product grouping from which the sample was drawn.
Beware, however, of three categories of sampling error:
- Bias (lack of accuracy)
- Dispersion (lack of precision)
- Non-reproducibility (lack of consistency)
These sampling errors are easily accounted for by knowledgeable practitioners.
How to Determine Minimal Sampling Sizes
Determinations of sample sizes for specific situations are readily obtained through the selection and application of the appropriate mathematical equation. All that is needed to determine the minimum sample size is to specify:
- If the data are continuous (variable) or discrete (attribute)
- If the population is finite or infinite
- What confidence level is desired/specified
- The magnitude of the maximum allowable error (due to bias, dispersion, and/or non-reproducibility)
- The likelihood of occurrence of a specific event
Serious About Samples (Quality Progress) Samples are most effective at catching defects when they are representative; however, sampling does not tell you where or when defects occur in the process, they only identify defects. In order to find and fix defects, other methods like statistical process control, control charts and control limits can be used.
Sample Wise (Quality Progress) Consider rules of thumb when selecting the correct sample size. The three most key components of sample size are how accurate or confident you need to be, how precise you need to be and the differences you are trying to measure.
Adapted from Quality Essentials: A Reference Guide from A to Z, ASQ Quality Press.