Acceptance Sampling of Veneer Using the Lot-Plot Plan

The lot-plot plan employs a frequency histogram in an analysis of quality on a lot basis and is based on sampling a variable rather than an attribute. The variables handled in this manner by the Jamestown Veneer and Plywood Corp., Jamestown, N.Y., were veneer thickness and moisture content. On each lot plot sheet is presented the histogram for a visual aid to understanding and simple calculations for estimates of mean and standard deviation. Estimates of upper and lower limits of the variable, three sigma either side of the mean, are made and indicated clearly on the histogram, along with the specification limits. The information obtained by use of the lot-plot led to a more suitable set of specifications, a reduction in out-of-specification goods, a minimum amount of inspection, a reduction in misunderstandings between vendor and purchaser, and the selection of more reliable vendors. The lot-plot method is one of the few tools of quality control that is readily understood by management. It is simple to operate with inexperienced help, has a high visual value, and lends itself to statistical analysis.

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