The Psychology of Quality and More

A Toolbook for Quality Improvement and Problem Solving (contents)

Pareto Chart: How to understand it

The Quality Toolbook > Pareto Chart > How to understand it

When to use it | How to understand it | Example | How to do it | Practical variations

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How to understand it

Given a set of recurring problems, it is unlikely that each problem will occur the same number of times in any one period. In fact, it is common that a few problems will occur far more often than the rest put together. This unequal distribution occurs in many situations and can be used to single out the 'vital few' from the 'trivial many'.

The Pareto Chart is simply a Bar Chart in which the bars sorted into size order, with the highest bar on the left, as below.

Fig. 1. The Pareto Chart

This not only shows the absolute priority of each bar, through its position in the chart, but also its relative priority, through its height as compared with the other bars.

As the Pareto Chart is often used for decision making, it is an important parts of building a Pareto Chart to identify the right item to measure and show on the chart, as different measures may well result in the bars be ordered quite differently.

In a stable process, the order of the bars may be expected to remain constant. Thus, if the order of the bars changes with successive measurements, this may indicate an unstable process (or an insufficient number of measurements). Improvements (i.e. changes in the process) will often result in the order of the bars changing. If the improvements are maintained, the new bar order will remain stable.

Pareto Charts may have different overall 'shapes' as shown in the table below. The 'spiky' Pareto Chart is the most useful, as it enables an easy selection of items to carry forwards for further action.

Table 1. Pareto Chart shapes

Shape of chart	Symptom	Possible problems
	Plateau	All bars are of comparable height. No clear selection of items.
	Convex	A number of bars on the left are of similar height. It is easier to reject those on the right than select from those on the left.
	Concave or Spiky	One or two bars are significantly higher than the rest (often making up 80% or more of the total). This is the ideal shape for selecting the vital few items for further action.

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