Six Sigma SPC - Statistical Process Control

Six Sigma SPC's Quality Control Dictionary and Glossary
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Random sampling

Technique that insures each item in a sample for inspection is selected completely by chance to be measured.

You should never do 100% inspection in any six sigma quality plan.

Sampling is more accurate in statistical process control for several reasons including it is less susceptible for 'inspection fatigue' in a process, because there is a lesser chance for errors. Products that are 100% inspected sometimes are found to have a astounding number of defects.

By using SPC and other control charts with a control plan, you can measure fewer production piece parts, (typically using a systematic sampling plan in unison with stratified sampling insuring that sampling bias does not become an issue), thus saving time and labor, and in theory, still have the same confidence level of the quality of you process.

Famous errors and lessons learned in random sampling can be attributed to Literary Digest magazine in 1936. They took a random sample from the telephone listings for a pre-election poll. The 10 million samples taken during the depression years were not accurate because in 1936 the people who could afford a phone and magazine subscription did not constitute a random sample. The company was soon out of business.

Also see systematic sampling, stratified sampling, and sampling bias.

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Last Updated: Thursday, 11-May-06 12:44:30 PDT