The seven basic tools of quality

by Matthew Moore on 30th November 2007

Pareto Charts

Based upon the Pareto Principle that states that 80% of a problem is attributable to 20% of its causes, or inputs, a Pareto Chart organises and displays information in order to show the relative importance of various problems or causes of problems. It is a vertical bar chart with items organised in order from the highest to the lowest, relative to a measurable effect: i.e. frequency, cost, time.

A Pareto Chart makes it easier to identify where the greatest possible improvement gains can be achieved. By showing the highest incidences or frequencies first and relating them to the overall percentage for the samples, it highlights what is known as the "vital few". Factors are then prioritized, and effort focused upon them.

An example of a Pareto Chart

Scatter Diagrams

A Scatter Diagram, or Chart, is used to identify whether there is a relationship between two variables. It does not prove that one variable directly affects the other, but is highly effective in confirming that a relationship exists between the two.

It is a graphical more than statistical tool. Points are plotted on a graph with the two variables as the axes. If the points form a narrow "cloud", then there is a direct correlation. If there is no discernible pattern or a wide spread, then there is no or little correlation.

If both variables increase as the other increases – i.e. the cloud extends at roughly 45 degrees from the point where the x and y axes cross – then they are said to be positively correlated. If the one variable decreases as the other increases, then they are said to be negatively correlated. These are linear correlations; they may also be non-linearly correlated.

Below is an example of a Scatter Diagram where the two variables have a positive linear correlation.

An example of a Scatter Diagram

Histogram

Like Pareto Charts, Histograms are a form of bar chart. They are used to measure the frequency distribution of data that is commonly grouped together in ranges or "bins". Most commonly they are used to discern frequency of occurrence in long lists of data. For instance, in the list 2, 2, 3, 3, 3, 3, 4, 4, 5, 6, the number 3 occurs the most frequently. However, if that list comprises several hundred data points, or more, it would be difficult to ascertain the frequency. Histograms provide an effective visual means of doing so.

"Bins" are used when the data is spread over a wide range. For example, in the list 3, 5, 9, 12, 14, 17, 20, 24, 29, 31, 45, 49, instead of looking for the occurrence of each number from 1 to 49, which would be meaningless, it is more useful to group them such that the frequency of occurrence of the ranges 1-10, 11-20, 21-30, 31-40 and 41-50 are measured. These are called bins.

Histograms are very useful in discerning the distribution of data and therefore patterns of variation. They monitor the performance of a system and present it in a graphical way which is far easier to understand and read than a table of data. Once a problem has been identified, they can then also be used to check that the solution has worked.

An example of a Histogram

Flow Chart

A flow chart is a visual representation of a process. It is not statistical, but is used to piece together the actual process as it is carried out, which quite often varies from how the process owner imagines it is. Seeing it visually makes identifying both inefficiencies and potential improvements easier.

A series of shapes are used to depict every step of the process; mental decisions are captured as well as physical actions and activities. Arrows depict the movement through the process. Flow charts vary in complexity, but when used properly can prove useful for identifying non-value-adding or redundant steps, the key parts of a process, as well as the interfaces between other processes.

Problems with flow charts occur when the desired process is depicted instead of the actual one. For this reason, it is better to brainstorm the process with a group to make sure everything is captured.