With statistical process charts (SPC), you have a statistical approach that is accessible and allows you to find solutions to system and process problems. One type of chart that falls under the SPC umbrella of tools is the rare event SPC.

When rare events occur in your industry, it is necessary to monitor them to ensure process stability and quality control. However, due to the rarity of the occurrences, traditional control charts are not your best monitoring solution.

Overview: What is a rare event SPC?

A rare event is an occurrence that happens at such a low frequency that it cannot be monitored with traditional control charts. A rare event statistical process control chart is a tool for charting these events successfully.

There are two statistical process control charts (SPC) that are utilized to monitor these types of events. One is called the G-chart, which is used to plot the opportunity number between these rare events or the number of days. The other is the T-Chart, which can be used for monitoring the time between rare events.

4 benefits of rare event SPC

There are some clear benefits to using these types of statistical process control charts in monitoring rare events.

1. Simplicity with a G-Chart

All that is needed in order to create a G-chart are the dates of the rare events or the opportunity number in between the occurrences. This makes the chart simple to create, understand, and utilize.

2. Statistical sensitivity

These types of charts offer a higher degree of statistical sensitivity than more traditional chart options.

3. Fewer occurrences are needed to create these charts

With options like P-Charts, a much higher number of occurrences would be needed to chart properly. With rare event charts, you also do not need to record and collect data on the full number of opportunities.

4. Less time investment

With a traditional chart, it might take months or years to have enough data in order to create a proper chart showing rare events. This is not necessary with rare event SPC.

Why is rare event SPC important to understand?

Rare event SPC is important to understand for the following reasons:

Effectiveness – Understanding these types of charts is important in that they are effective in keeping track of rare events.

Detection – Having a solid foundation in these types of charts can help you better detect instability.

Knowing which chart to use and when – Having an understanding of these charts and the differences between them allows you to know when it is best to use one type of rare event chart as opposed to another. For example, when to use what instances are best to use the T-Chart instead of a G-Chart.

An industry example of rare event SPC

At a manufacturing plant, there is an annual review of serious accidents that have occurred in the machine shop. With these types of situations being so rare, using a traditional charting method proves to be inefficient. Instead, the person that drafts the review opts to utilize a rare event statistical process control chart known as a G-Chart.

3 best practices when thinking about rare event SPC

Here are some key practices to keep in mind when it comes to rare event statistical process control charts:

1. Know which chart is best in a given situation

While you may use a G-Chart in order to monitor opportunities or days between events, a T-Chart can be used instead if the data is continuous. Alternatively, if you are trying to use a T-Chart when you have the dates for each rare event but not the time, you should instead create a G-Chart.

2. Use these to determine process stability

If you need to track if your rare events are an indicator of a process that is not stable and therefore in need of attention, utilize these types of control charts.

3. Be aware of what a G-Chart assumes

You should be aware that a G-Chart assumes that there is a reasonable amount of constancy to the occurrences.

Frequently Asked Questions (FAQ) about rare event SPC

What does the G stand for in G-Chart?

The G stands for geometric.

What are some indicators that a traditional chart will not be effective?

One indicator would be when the amount of data with the numerator of zero is beyond 20%. Another indicator is when control limits get too close to the average due to the denominator of the control chart being excessively large. It also may be necessary if the lower control limit is not present.

In which industries are these types of charts used?

These types of control charts are used in the medical field, manufacturing, service, and much more.

Rare event SPC

Across many industries, being able to use the G and T rare event statistical process control charts are an integral part of managing quality control and improvement efforts.

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