The goal is to determine the distribution or variation of a set of data points in chart form (see more in George and Maxey 2004). Figure 5.8 shows a typical histogram.


  • • Histograms make it possible to understand the variations of a particular process at a glance.
  • • The shape of the histogram will show the behavior of the process.
  • • The shape and size of the distribution reveals the sources of hidden variation.
  • • They help to determine the capacity of a particular process.
  • • They act as a starting point for the development process. How to implement?
  • 1. Collect data, 50-100 data points.
  • 2. Select the data range.
  • 3. Calculate the size of the class range.

Figure 5.8 Histogram.

  • 4. Divide the data points into classes and determine the class limit.
  • 5. Count the data points in each class and draw the histogram.

Distribution Diagrams

A distribution diagram shows a correlation between the two variables in a certain process. The data points have been distributed on a diagram. The goal is to uncover the correlation that may be present between the quality characteristics and the factors that may lead to them (see more in George and Maxey 2004). Figure 5.9 shows a typical distribution diagram.

How to implement?

  • 1. Decide which paired factors to examine. Both can be measured on a progressive linear scale.
  • 2. Collect between 30 and 100 paired data points.
  • 3. For both variables, find the highest and lowest values.
  • 4. Draw the vertical (y) and horizontal (x) axes of a graph.
  • 5. Organize the data.

The shape that the point series will take will give you some clues regarding the relationship between two variables that you have tested.

Control Sheets

Control sheets are used for collecting and editing the data so measured or calculated. The collected data can be used as information input for other quality tools (see more in George and Maxey 2004). Figure 5.10 shows an example control sheet.

Distribution diagram

Figure 5.9 Distribution diagram.

Control sheets—example

Figure 5.10 Control sheets—example.


  • • Control sheets collect data in a systematic and orderly manner.
  • • They determine the sources of the problem.
  • • They ease the classification of data.
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