Statistical Methods for Quality Improvement, Third Edition

CHAPTER 2

Basic Tools for Improving Quality

There are various statistical and nonstatistical tools that have been used extensively in quality improvement work. In particular, there are seven simple tools that have often been referred to as “the seven basic tools,” with the late Kaoru Ishikawa generally associated with the term. In particular, see Ishikawa (1976). The tools are:

1. Histogram

2. Pareto chart

3. Scatter plot

4. Control chart

5. Check sheet

6. Cause-and-effect diagram

7. Defect concentration diagram

The first four of these are statistical/graphical techniques. They are introduced here and some are covered in greater detail in subsequent chapters. The last three tools are discussed only in this chapter. It is important to realize that although many gains have been made using just these seven tools, there are other tools, such as experimental designs (see Chapter 13), that should additionally be used. See also the “seven newer tools” that are discussed in Section 2.8.

2.1 HISTOGRAM

A histogram is a bar chart that shows the relative frequencies of observations in each of several classes. For example, Figure 2.1 is a histogram that might represent quality control data that have been grouped into seven classes, such as values of a process characteristic that have been obtained over time. A histogram is thus a pictorial display of the way the data are distributed over the various classes. As such, it can indicate, in particular, whether the data are distributed symmetrically ...