Effect of the resolution of measurements in the behavior of the shewhart control charts for means

PDA J Pharm Sci Technol. 2010 Nov-Dec;64(6):562-73.

Abstract

Measurements of limited resolution change the behavior of a Shewhart average chart in two ways. The standard deviation of measurements differs from process standard deviation due to rounded values and discreteness of values of subgroup averages. The probability of getting a point in control depends on the position of accessible values relative to the position of control limits. In many processes of the pharmaceutical industry, Shewhart control charts are very useful tools. In this paper, one example about the use of these charts in the abovementioned industry is given. Resolution requirements of a measurement system for successful operation of an average Shewhart control chart are presented. Recommendations are given for acceptable criteria about the resolution of the measurement system when an average control chart is implemented, and are compared with already existing literature.

Lay abstract: It is vital for the pharmaceutical industry to deliver safe medicines to patients. Quality control is a part of the quality management that is oriented to control raw materials, processes, and finished products. This constitutes a usual activity of pharmaceutical companies that use for that purpose different chemical, physical, and biological assays. Any control method will use measurement devices. The measurement devices require a certain measurement capability and must be fit for the purpose of controlling the quality of pharmaceutical products. The equipment capability includes, among other elements, the resolution, which means the smallest change in a quantity being measured that causes a perceptible change in the corresponding indication. Control charts have been extensively used for statistical process control and for its proper operation, measurement equipment of the adequate resolution is needed. Based on a case of the pharmaceutical industry and simulation tools, this paper studies the proper resolution of measurement devices for use in control charts.

Publication types

  • Case Reports

MeSH terms

  • Humans
  • Probability*
  • Quality Control*
  • Quality of Health Care