Excel Control Charts (Table of Contents)
Introduction to Control Charts in Excel
Control charts are statistical visual measures to monitor how your process is running over the given period of time. Whether it is running as expected or there are some issues with it. There are important tool under Statistical Process Control (SPC) which measures the performance of any system/processes whether they are running smooth or not. If there are any disturbances, the processes can be reset. Control charts are most of the times used under manufacturing processes, in order to check whether the manufacturing processes are under control or not.
Definition of Control Chart
A control chart is nothing but a line chart. It can be generated when we have upper and lower control limits present for the data and we wanted to check whether the control points are lying between the actual upper and lower limits or going out of those. If the control points are lying well within limits, then the process is said to be in control. If some of the points are lying outside of the control limits, the process is said to be not in control. Though there are different Statistical Process Control (SPC) software available to create the control charts, Microsoft Excel does not lack in creating such charts and allows you to create those with more ease. In this article, we are about to see, how control charts can be created under Microsoft Excel.
Example of Control Chart in Excel
Suppose we have a data of 30 observations from a manufacturing company as below. We want to see whether the process is well within the control limits or not. We will draw a Control chart in order to see visually whether the process is in control or not. See the screenshot of the partial data given below.
Step 1: In cell F1 apply the formula for “AVERAGE(B2:B31)” where the function computes the average of 30 weeks.
After applying the above formula answer is shown below.
Step 2: In cell G2, apply the “STDEV.S(B2:B31)” formula to calculate the sample standard deviation for the given data. This formula calculates the sample standard deviation. We have a different formula in order to calculate the population standard deviation in excel.
After applying the above formula answer is shown below.
Step 3: In column C called Control Line, go to cell C2 and input the formula as =$G$1. The $ sign used in this formula is to make the rows and columns as constants. It means when you drag and fill the remaining rows for column C, all cells will be having the same formula as the one imputed in cell C2. Drag and fill the remaining cells of column C. You’ll be able to see the output as below.
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After applying the above formula answer is shown below.
Drag and fill the remaining cell of column C.
Because, the Control Line is nothing but the line of center for control chart which does not change over observations, we are taking Average as a value for Control Line.
Step 4: For Upper Limit, the formula is. Therefore, in cell D2, put the formula as =$F$2+(3*$G$2). Again, the upper limit is fixed for all the week observations. Therefore we have used the $ sign to make rows and columns constant. Drag and fill the remaining cell of column D and you’ll be able to see the output as below.
After applying the above formula answer is shown below.
Drag and fill the remaining cell of column D.
Step 5: Lower Limit for control chart can be formulated as in cell E2, put the formula as =$G$1-(3*$G$2). This formula calculates the lower limit which is fixed for all weekly observations the $ sign achieves that in this formula. Drag and fill the remaining cells with formula and you’ll be able to see the output as below.
After applying the above formula answer is shown below.
Drag and fill the remaining cell of column E.
Explanation:
The Upper Limit, Lower Limit, and Central/Control Line are the parameters of the control chart. We calculate these terms because we have a theory base for that. In Statistical Process Control (SPC), we say that the processes are going normal if 99.73% observations are scattered around the Central/Control Line within 3 standard deviations above and below the same (that’s why we calculate upper limit as 3 standard deviation above from average which is central line and lower limit as 3 standard deviations below of the average). If it happens, then and only then we can say that the process is following the normal pattern. Otherwise, the process is said to be behaving abnormally and we need to make the adjustments among the machineries.
We are done with the required information which is needed to plot control chart in excel. Now we will head towards adding a one in excel.
Step 6: Select the data from column A and B (spread across A1:B31) from your excel sheet and go to Insert tab present at the excel ribbon. Under Charts section navigate towards Insert Line and Area Chart button.
Step 7: Press the Insert Line or Area Chart dropdown button, you’ll be able to see a handful of line and area chart options available under excel. Out of those all, under 2 – D Line section select Line with Markers and Press the Enter key.
Your graph will look like below after removing weeks data from the line chart.
Now, we would like to add the central/control, lower and upper limit lines to this chart so that we can see how the weekly data is moving.
Step 8: Right-click on the Graph and click on the “Select Data” option.
A “Select Data Source” dialog box will open up and click on the “Add” button.
Step 9: Inside Legend Entries (Series), after clicking on “Add” button and input Control Line as a “Series name” and corresponding control line values as a “Series values” under “Edit Series” dialog box. Click on the “OK” button once done.
Step 10: After clicking on “Add” button and input Upper Limit as a “Series name” and corresponding Upper Limit values as a “Series values” under “Edit Series” dialog box, click “OK” button after done with it.
After clicking on “Add” button and input Lower Limit as a “Series name” and corresponding Lower Limit values as a “Series values” under “Edit Series” dialog box, click “OK” button after done with it.
You’ll be able to see the control chart ready as below.
Step 11: Give the title as “Control Chart” for this graph and you are done with it.
This is how we can create a control chart under Excel. This is from this article. Let’s wrap the things up with some points to be remembered.
Things to Remember
- Control charts are plotted to see whether the process is within the control or not.
- It is mandatory to calculate and plot Central/Control Limit, Upper Limit, and Lower Limit in order to check whether the process lies between them.
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