Reduction of Sampling Inspection Using Variables Data in the Presence of Batch-To-Batch Variability

• Activity Number: 37
• Type: Contributed
• Date/Time: Sunday, August 3, 2014 : 2:00 PM to 3:50 PM
• Sponsor: Quality and Productivity Section
• Author(s): David Trindade*+
• Companies: Bloom Energy
• Keywords: variables sampling plans ; attributes sampling plans ; batch to batch variability ; quality inspection ; manufacturing quality assurance

Abstract

Many companies perform attribute sampling in which a fixed number of units is inspected and each unit classified as either conforming or non-conforming to single or double sided specification limits. An acceptance number determines the lot disposition to accept or reject. Attribute plans are simple in concept involving only counts. Variables plans require the measurement of a quality characteristic. Variables plans are based on the distance of the variables from specification limits and offer many benefits including the same discriminating ability as inspection by attributes but with smaller sample sizes, a reduction in time and costs to perform measurements, and greater efficiency in the use of company resources. However, there are several critical assumptions and factors involved in variables sampling including batch to batch variability, independence, and normally distributed variables. In this presentation, we describe a successful implementation, along with methods for verification of assumptions, of variables sampling in a manufacturing environment. The use of variables sampling has resulted in a dramatic reduction of inspection requirements and increased productivity.