Tools, Technologies and Training for Healthcare Laboratories

Six Sigma Design and Error Budgets

If the concept of Six Sigma seems too difficult or advanced, perhaps it's easier to think of something we all do: budgeting. Whether we're making a budget at home in our personal life, or at work for a company or department, that process of assessment and adjustment is very close to the process we need to do with error and quality. (Preview)

The quality design tool recommended here for Six Sigma applications is the chart of operating specifications, or OPSpecs chart. The previous lesson described the graphical attributes of the OPSpecs chart, which include the abilities to

  • assess the process capability, or sigma level, on the basis of the CV and Bias observed for a method and
  • relate the sensitivity or error detection of a QC procedure to the CV and Bias that are allowable for different control rules and different numbers of control measurements.

The graphical demonstration of these properties shows the usefulness of the OPSpecs chart for establishing performance specifications and QC procedures in applications where analytical tolerance limits are defined in terms of the allowable total error for a laboratory test.

The objective in this lesson is to describe the framework for constructing quantitative relationships between the factors that affect the variability of a test result and the analytical and clinical tolerance limits for a testing process. By understanding this framework, you will see how the OPSpecs chart can be extended to clinical applications where a tolerance limit is defined in the form of a medically important change in test results or a clinical decision interval (Dint). Clinical tolerance limits encompass pre-analytic variables (subject and/or biologic variation, sample variation, specimen bias) as well as analytic variables (CV, bias, control rules, N). The rationale for handling the clinical type of quality requirement can be demonstrated by analogy with budgets, in this case, error budgets for laboratory testing applications.


Six Sigma, 2nd EditionWant to read more of this article?

We invite you to purchase the Six Sigma QC Design and Control manual, Second Edition, available at our online store. You can also download the Table of Contents and a Sample chapter.