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Analyzing a Process Before and After Improvement: Historical Control Charts with Stages

We tend to think of control charts only for monitoring the stability of processes, but they can be helpful for analyzing a process before and after an improvement as well. Not only do control charts allow you to monitor your process for out-of-control data points, but you’ll be able to see how your process mean and variability change as a result of the improvement.

Control Chart

You might create separate before and after control charts for each phases of the improvement project, but making comparisons between those charts can be difficult. You could also try analyzing all of the data over the course of your project in a single control chart, but this could result in incorrectly flagging out-of-control points. This method also won’t calculate changing mean and control limit values.

Putting Control Charts on the Stage

The best choice in a case like this is to create a control chart in “stages,” which is easy to do with a statistical software package such as Minitab. Stages are used to create a historical control charts that shows how a process changes over specific time periods. At each stage, Minitab recalculates the center line and control limits on the charts.

Check out this simple example featuring a control chart that tracks admission times for a hospital’s ICU over a three-month period:

Minitab Control Chart

Though the process for admitting patients underwent improvement each month, all of the data is graphed on a single chart without utilizing stages.

It’s easy to see that the admission times decreased, but the only thing we really know for certain is that the mean admission time was about 18 minutes. Also, quite a few points at the outset suggest that this process is out of control.

Take a look at the exact same data, now charted with stages:

Minitab Control Chart

At each stage, Minitab recalculates the center line and control limits on the chart. Now it’s easy to see that this process was kept in control at every stage of improvement, and you also get a much more accurate idea of how the mean admission times went down at each stage of the improvement. You can also see that the variation of the process—the area between the upper and lower control limits—decreased over time.

To create a historical control chart with stages in Minitab, you’ll need a column of data indicating the stage of each observation. Here’s what that looks like for our ICU example:

Minitab Worksheet

The column "month" contains the stages and the values June, July, and August correspond to each stage in the process. When you are creating the chart, just click the chart options button and click the stages tab. There you can enter the column containing the stages information. In this case, the column month contains the stages:

Minitab Stages

A special thanks to Patrick North in Minitab’s Information Development department for his contributions to this blog post!

For more on control charts, check of these posts:

Control Charts Show You Variation that Matters

Control Charts: Subgroup Size Matters

How to Create and Read an I-MR Control Chart

7 Deadly Statistical Sins Even the Experts Make

Do you know how to avoid them?

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Comments

Name: Lisa P • Monday, February 3, 2014

Entering dates for stages is more complicated than described here. My initial attempts failed. Entering the date in quotes as Minitab Help says to, didn't help. The problem turned out to be that the dates were originally in date/time format even though they were displayed as MM/DD/YY. I was able to get around this by using the sequential batch number associated with the process change instead of the date.


Name: Carly Barry • Monday, February 3, 2014

Hi Lisa - I reached out to our tech support group to see if they might have any tips/tricks when it comes to entering dates for stages. They responded with some good info (too long for this comment box) that I sent to the email address you gave when you commented. Hope this helps!

Thanks for reading!
Carly


Name: Debra Grossbach • Wednesday, May 28, 2014

I am also having problems with entering dates for stages, and I haven't yet come up with a solution. Since my data set was relatively small I created a column that labeled each set of data as Process Stage 1 and 2, but I still couldn't it to work.


Name: Carly Barry • Monday, June 2, 2014

Hi Debra - I followed up with you directly at the email you listed when you commented here. Thanks for reading!

Carly


Name: Pawell • Thursday, June 26, 2014

Hi all, stages show me three I-MRs in one window - that's nice... How can I get three I-MRs in three windows (analogy to "by variables" in Graphical Summary) in "one click"?


Name: Carly Barry • Thursday, June 26, 2014

Hi Pawell - Unfortunately, there is no direct option in Minitab to use a by variable in control charts. You would have to create each control chart individually. Let me know if you have any follow-up questions. Thanks for reading!

Carly


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