## Choosing the right type of control chart

First of all, please note that we'll keep the things simple. Our aim is to make able to get a control chart in minutes even if you are completely new to statistical process control and we will not go into the details. We are also cutting all the details regarding the data collection process. However, we still encourage you to take the time to follow formal training or read about the six sigma theory. I recommend the book 'The Executive Guide to Understanding and Implementing Lean Six Sigma'. It is a great ressource to learn about the methodology and to drive successful six sigma initiatives !Several types of control charts exist. The selection of the right type of chart is based on the type of observed characteristic. We can differentiate 2 main categories :

- Control charts for measurements and variables
- Control chart for counts and attributes

### Measurements and variable control charts

To make it simple, you can use these control chart if collecting data involves the measurement of a continuous variable such as a length, time or temperature

- X-chart
- MR Chart
- X-bar chart
- Range chart
- S-Chart

The X-chart is for individual measures, eg if you periodically measure the weigth of one single part. Each point represents the measure recorded. It is typically used in combination with a MR chart.

The 'Moving range' chart is used in combination with the X-Chart. It plots the difference between 2 consecutives points.

The X-bar chart is used if each subgroup is a sample of several units, parts individuals,.... eg if you are periodically taking samples of several parts and measuring a weight. It is typically used in combination with a Range chart or a S-Chart

The 'range' chart or R-chart has to be used in combination with the X-bar Chart. It plots the range of the each subgroup.

The 'S' chart or R-chart has to be used in combination with the X-bar Chart. It plots the standardeviations of the each subgroup.

### Attribute control charts

You can use these charts if the data collection involves counting or calculating a proportion. For instance, it can be a number of defect per unit, a number of products returned per unit of time,...

- np chart
- c chart
- p Chart
- u chart
- g Chart

The np chart is used when the size of the subgroup is constant and you count individuals with a characteristic of interest from a sample, for instance the number of nonconforming units. For each individual you count 0 or 1 and the total count result can't exceed the number of samples.

The c chart is used if the size of the subgroup is constant and you count occurrences of a characteristic of interest, for instance if you count the total number of defects (there may be more than one defect per unit).

The p chart is used if the size of the subgroup is variable and you count individuals with a characteristic of interest from a sample, for instance the number of nonconforming units.

The u chart is used if the size of the subgroup is constant and you count occurrences of a characteristic of interest, for instance if you count the total number of defects (there may be more than one defect per unit.

The g chart is used if you count the number of events between occurences of one -usually rare - particular event. For instance, it may be used to count the flights without accident between plane accident

## Creating a control chart

On the home page :1- Select the type of control chart

2- Download the excel template and replace the data provided as an example by your own data. You can include as many lines as you want, or add as many columns as needed to adapt the size of subgroup you are using.

3- Upload your data

... And you are done with these 3 simple steps.

You may even the second steps if your data is organized as needed. The expected format does not change, so may take use it directly from the data collection steps.

## Interpreting a control chart

Work in progress. Come back soon!Control charts are a useful tool but it has to be used as part of consistent management approach. We recommend the book 'The Executive Guide to Understanding and Implementing Lean Six Sigma'. It is a great ressource to support a six sigma approach !