Chapter 6. Understanding Process Variation with Control Charts

In this chapter, we will cover the following recipes:

  • Xbar-R charts and applying stages to a control chart
  • Using an Xbar-S chart
  • Using I-MR charts
  • Using the Assistant tool to create control charts
  • Attribute charts' P (proportion) chart
  • Testing for overdispersion and Laney P' chart
  • Creating a u-chart
  • Testing for overdispersion and Laney U' chart
  • Using CUSUM charts
  • Finding small shifts with EWMA
  • Control charts for rare events – T charts
  • Rare event charts – G charts

Introduction

Control charts are very simple graphical tools which show us if measurements/results are stable over time. They look at the mean and variation of the data and check to see whether the observed data shows any patterns that would not be expected to occur if the data was purely random. This special cause variation is indicated by tests that look for these patterns. They are based on there being a low probability of these patterns occurring randomly.

Minitab provides a wide range of control charts for different scenarios. These include the standard control charts for monitoring a process over time such as Xbar-R or I-MR charts as well as multivariate control charts and charts to plot rare events. Now, we will look at using some of the more traditional charts, but also show the use of some of the newer charts in Minitab.

The Control Charts menu is separated into different submenus to make it easier for the user to select the correct chart. The following screenshot shows us where to find the Control Charts section within the Stat menu and the choices presented by the sub menus:

Introduction
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