Measure Phase And Statistical Charting

Introduction

The measure phase in six sigma for improvement projects quantitatively evaluates the current or default system KIVs, using thorough measurements of key output variables (KOVs) before changes are made. This information aids in evaluating effects of project-related changes and assuring that the project team is not harming the system.

Statistical charting:

1. The term “measurement systems” refers to the methods for deriving KOV numbers from a system, which could be anything from simple machines used by an untrained operator to complicated accounting approaches applied by teams of highly trained experts.
2. The terms “gauge” and “gage,” alternate spellings of the same word, referred historically to physical equipment for certain types of measurements. However, here gauge and measurement systems are used synonymously, and these concepts can be relevant for such diverse applications as measuring profits on financial statements and visually inspecting weld quality. Measurement systems generally have several types of errors that can be evaluated and reduced.
3. The phrase “gauge repeatability & reproducibility” (R&R) methods refers to a set of methods for evaluating measurement systems. This chapter describes several gauge R&R related methods with examples. Thorough evaluation of system inputs generally begins after the acceptable measurement systems have been identified.
4. Evaluation of systems must include sufficient measurements at each time interval to provide an accurate picture of performance in that interval. The evaluation must also involve a study of the system over sufficient time intervals to ensure that performance does not change greatly over time.
5. The phrase “statistical process control” (SPC) charting refers to a set of charting methods offering thorough, visual evaluation of system performance and other benefits described later in this chapter. The primary purpose of the measure phase for design projects is to systematically evaluate customers’ needs.
6. Therefore, it is helpful to study similar systems using gauge R&R and control charts. In addition, the measure phase can also include techniques described in the context of other phases that focus attention on customer needs, such as cause & effects matrix methods.

Example (Gauge R&R and SPC Charting) Describe the relationship between gauge and SPC charting?

Answer: Gauge R&R evaluates measurement systems. These evaluations can aid in improving the accuracy of measurement systems. SPC charting uses measurement systems to evaluate other systems. If the measurement systems improve, SPC charting will likely give a more accurate picture of the other systems quality.

Evaluating Measurement Systems:

1. The phrase “standard values” refers to a set of numbers known to characterize correctly system outputs with high confidence and legible errors.
2. Standard values effectively constitute the true measurements associated with manufactured units and are believed to be the true values within the explicit or implied uncertainty. For example, a set of units of varying lengths are made using
3. an alloy of steel with low thermal expansion properties.
4. The standard values are then the believed true length values in millimeters at room temperature, found at great expense at a national institute of standards. The phrase “measurement errors” refers to the differences between the output variable values derived from a measurement system and the standard values.
5. This section describes several methods for characterizing the measurement errors of measurement systems.
6. In some cases, measurement errors of measurement systems will be found to be acceptable, and these systems can in turn be trusted for evaluating other systems. In other cases, measurement systems will not be deemed acceptable.
7. The improvement of measurement systems is considered to be a technical, investment related matter beyond the scope of this book. Once improvements in the measurement systems have been made, however, the methods here can be used to evaluate the progress.
8. It is likely that the associated measurement systems will eventually become acceptable. Fortunately, many of the other methods in this book can still give trustworthy results even if measurement systems are not perfect.