The Seven Basic Quality Tools: When to Use Pareto, Fishbone and Control Charts

What are the seven quality tools, and why do they still matter?

Kaoru Ishikawa highlighted these tools in his guide to quality control because they let front-line staff solve most everyday problems without advanced statistics. The American Society for Quality (ASQ) lists seven: the cause-and-effect diagram, check sheet, control chart, histogram, Pareto chart, scatter diagram and stratification. Some newer lists replace stratification with the flowchart. ^[1]

Their value today is as a shared language: when a team in a hospital or government entity shows a Pareto chart, everyone sees where the biggest problem lies without a long explanation.

Which tool do you use, and when?

A common mistake is for a team to start drawing a fishbone before it knows which problem deserves analysis. Always start with a clear question, then choose the tool that answers it.

Check sheet and stratification: correct data first

A check sheet is a simple form for tallying how often each type of problem occurs. ^[1] It looks obvious, but its value lies in agreeing definitions: what counts as “late”? More than one working day, or beyond the published service time? If each employee records in their own way, every later analysis is built on sand.

Stratification means separating data by groups such as branch, shift or service type. ^[1] An error rate may look acceptable overall, then turn out after stratification to be high on the night shift alone.

Pareto: where to focus first

A Pareto chart ranks problems or causes from largest to smallest, based on the 80/20 rule: a few causes explain most of the effect. ^{[1, 3]} ASQ summarises how to build one: choose the categories, then the measure (count, cost or time), then the time period; collect the data; arrange the bars in descending order, grouping small categories as “other”; and, if you wish, add a cumulative percentage line. ^[3]

Watch the choice of measure. The most frequent cause may not be the most costly, so you may need one Pareto chart by number of cases and another by cost before setting priorities.

Fishbone: organising possible causes

The cause-and-effect diagram, or fishbone, puts the problem at the head and arranges its possible causes in main and secondary branches. ^[1] In services, branches might be people, procedures, systems, environment, measurement and inputs.

Remember that the diagram collects possible causes, not proven ones. You then need data to confirm or rule out each cause, which is where the scatter diagram, histogram or a return to the check sheet comes in.

Histogram and scatter diagram: understanding the data

A histogram shows the shape of the data’s distribution more clearly than a table. ^[1] If transaction times have two peaks, there may be two different processes operating under one name.

A scatter diagram plots two variables together to reveal a possible relationship, such as daily request volume and reply time. ^[1] But correlation does not prove causation; a third factor may be driving both.

Control chart: is the process stable?

A control chart plots a measured value over time against control limits calculated from the process’s own data. It separates two kinds of variation: common causes inherent in the process, and special causes arising from unusual events that make the process unstable and unpredictable. ^[2]

Out-of-control signals listed by ASQ include a point beyond the control limits, eight points in a row on one side of the centre line, or an obvious persistent pattern. The team then looks for the special cause and documents it. ^[2] If the process is stable but its performance is unacceptable, the answer is to change the process itself, not to chase each point.

A hypothetical example: complaints at a customer service centre

A service centre in a government entity is receiving more complaints. The team designs a check sheet of complaint types and collects four weeks of data, stratified by service channel. A Pareto chart shows that three types account for most complaints, led by slow replies to email.

The team draws a fishbone for that type, and possible causes include manual email routing and no reply templates. A scatter diagram confirms that reply time rises with the number of messages per shift. After introducing automatic routing and reply templates, the team tracks weekly average reply time on a control chart to confirm the improvement has held. The case is hypothetical, to illustrate the sequence.

Mistakes that spoil the analysis

• Drawing the tools with data collected under different definitions.
• Treating fishbone causes as facts without checking them.
• Treating every rise on the chart as a crisis when it is within normal variation.
• Putting the chart in a report without a decision or action following it.

Conclusion

Start with a question, then choose the tool that answers it: a check sheet to collect data, Pareto to set priorities, a fishbone to organise causes, and a control chart to tell whether an improvement has held.