Title page for ETD etd-041999-130738

Type of Document Dissertation
Author Hughes, Christopher Scott
Author's Email Address chhughe3@vt.edu
URN etd-041999-130738
Title Variable Sampling Rate Control Charts for Monitoring Process Variance
Degree PhD
Department Statistics
Advisory Committee
Advisor Name Title
Arnold, Jesse C. Committee Co-Chair
Reynolds, Marion R. Jr. Committee Co-Chair
Coakley, Clint W. Committee Member
Foutz, Robert Committee Member
Ye, Keying Committee Member
  • Shewhart chart
  • integral equation
  • Markov chain
  • CUSUM chart
  • EWMA chart
Date of Defense 1999-04-13
Availability restricted
Industrial processes are subject to changes that can adversely

affect product quality. A change in the process that increases

the variability of the output of the process causes the output to

be less uniform and increases the probability that individual

items will not meet specifications.

Statistical control charts for monitoring process variance can be

used to detect an increase in the variability of the output of a

process so that the situation can be repaired and product

uniformity restored. Control charts that increase the sampling

rate when there is evidence the variance has changed gather

information more quickly and detect changes in the variance more

quickly (on average) than fixed sampling rate procedures. Several

variable sampling rate procedures for detecting increases in the

process variance will be developed and compared with fixed

sampling rate methods.

A control chart for the variance is usually used with a separate

control chart for the mean so that changes in the average level

of the process and the variability of the process can both be

detected. A simple method for applying variable sampling rate

techniques to dual monitoring of mean and variance will be

developed. This control chart procedure increases the sampling

rate when there is evidence the mean or variance has changed so

that changes in either parameter that will negatively impact

product quality will be detected quickly.

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