Industrial Statistics with Minitab

Part Three

MEASUREMENT SYSTEMS STUDIES AND CAPABILITY STUDIES

When measuring a characteristic of a product it is sometimes forgotten that the whole measurement process is subject to variability. There are no perfect measuring devices, nor do people involved in the measuring process (operators) always behave in the same way. The result is that, inevitably, the available data are contaminated by the variability introduced by the measurement system.

The good news is that we can design a data collection plan that actually allows separating the variability due to parts of that introduced by the operators and the measuring device. These studies are called: repeatability studies (variability of the device) and reproducibility studies (variability introduced by metering or by environmental factors). Minitab is helpful for this type of study and Chapter 13, the first chapter of Part 3, is dedicated to that issue.

The next two chapters (14 and 15) are devoted to capability studies. They are aimed at measuring the natural variability of a process when it is solely affected by what are called ‘random variability causes’. They are characterized by being many, inevitable, small and almost imperceptible one by one. However, when acting together, they provoke this inevitable variability that we call capacity.

Two types of capability are of interest: short and long term. Short-term capability is measured in a short space of time, generally by measuring units produced in a row, one after another; ...