An SPC (Statistical Process Control) control chart is a graphical representation of process data over time, used to detect any potential out-of-control conditions or assignable causes of variation in a process. In this lesson, you will learn how to create a statistical process control chart.
What is a control chart?
SPC control chart is a graphical representation of process data over time that helps in identifying and monitoring any potential out-of-control conditions or assignable causes of variation in a process. The purpose of an SPC control chart is to determine whether a process is in statistical control, meaning that the process is operating consistently and producing results within predictable limits, or if it is not in control, meaning that there are assignable causes of variation present that need to be addressed.
SPC control charts are commonly used in quality control and process improvement, particularly in manufacturing and other industrial processes, to monitor process performance and identify opportunities for improvement. There are several types of SPC control charts, including X-bar and R charts, X-mR charts, I-MR charts, and individual and moving range charts, each designed to monitor different aspects of a process.
By tracking process data over time and comparing it to control limits, SPC control charts provide valuable insights into process performance, enabling organizations to make informed decisions about process improvements and quality control efforts.
With an spc chart, you are able to follow how your process is evolving over time.
In most cases, the line chart type is selected for the control charts. Two lines of a lower control limit and an upper control limit are added to the chart to represent the limit values. On the control chart, you can check whether the process is in the state of statistical control. The line on the control chart must remain between these control limits.
If the values in the graph are outside the lower or upper range, it means that statistically the process is out of control. In this case, immediate steps should be taken. After that, the process should be reset.
How to create a control chart?
First, we are going to find the mean and standard deviation. To find the mean click on the Formula tab, click on More Function select Statistical and then Average from the dropdown menu. Select cells B2 to B20 and press okay.
Click on the Formula tab, click on More Function select Statistical and then STDEV.S from the dropdown menu. Select cells B2 to B20 and press okay.
Select cell C1 and type "Mean (CL)" in it. Select C2 and type "=I$1". Move your mouse to the bottom right of the cell until a black plus sign appears. Drag the plus sign all the way to cell C20 to copy the mean.
Select cell D1 and type "UCL" in it. Select D2 and type "=I$1+ (I$2*3)". Move your mouse to the bottom right of the cell until a black plus sign appears. Drag the plus sign all the way to cell D20 to copy the mean.
Note: UCL= upper control limit typically is 3 standard deviations away from the average which is mean+3 times the standard deviation.
Select cell D1 and type "LCL" in it. Select E2 and type "=I$1+ (I$2*3)". Move your mouse to the bottom right of the cell until a black plus sign appears. Drag the plus sign all the way to cell E20 to copy the mean.
Note: LCL= lower control limit typically is 3 standard deviations away from the average which is mean-3 times the standard deviation.
SELECT Sample Measures, Mean(CL), LCL and UCL.
Click on Insert tab, click on Line Chart and then Click on Line.
You have created your chart. Resize it. Remove the small black lines by double clicking on them and pressing Delete. That's it, you're done.
This is what your final chart will look like.
Your Statistical process control chart is ready. Tee process is under control as sample measures are not even close to the both lower and upper control limits.
Note that these steps are just a general guideline and that the specific steps and calculations used to create an SPC control chart may vary depending on the type of control chart used and the methodology followed. It's important to consult a knowledgeable statistician or quality engineer for guidance on creating an SPC control chart for a specific process.