How to create an SPC chart and how technology can help

As we’ve seen in previous articles in this series, creating, and preparing to create an SPC chart – or a Statistical Process Control chart - can be a lengthy and involved process, but one that ultimately can bring great reward to your Quality Improvement (QI) programme. Used widely by the NHS to help understand whether improvement has come about by change, the NHS says that SPC is a methodology that ‘provides an easy way for people to track the impact of improvement projects.'

In this article we’re going to look at how you can create an SPC chart manually, using templates such as Excel and using other software to help. Let’s start by looking manual SPC charts.

Creating an SPC chart manually – calculating control limits

Let’s go back and revisit the definition of statistical process control and why it’s so popular in the NHS. SPC is described as a ‘a good technique to use when implementing change as it enables you to understand whether changes you are making are resulting in improvement — a key component of the Model for Improvement widely used within the NHS.’

A key part of Statistical Process Control is the control chart. We’ve looked at elements in previous articles – and although they are complex they are ultimately well worth the effort.

Of course, you can create an SPC chart manually. However, we wouldn’t necessarily recommend it – as you will read later on! We might recommend that you create areas of your control chart manually, for example data points. ‘It is best to plot the data points manually in the early stages of making an SPC chart. Once the formulas and meaning is understood, you can use statistical software to update them.’

You could also potentially look at manually calculating control limits. Control limits, sometimes known as sigma limits, ‘define the boundaries of expected common cause (random) variation around the mean’. Control limits mean that you don’t waste time looking for issues in the process when there might not be any. Here’s how you can calculate control limits’:

• Estimating the standard deviation, σ, of the sample data
• Multiplying that number by three
• Adding (3 σ to the average) for the UCL and subtracting (3 σ from the average) for the LCL’

If this is something you’d find easier doing manually – it’s quite OK to do so. Although we’d always recommend using software to carry out the whole SPC chart process.

That said, one of our previous blogs ‘What is Statistical Process Control?’ advises: ‘For many years there has been a high expectation that the person creating the charts fully understood the detailed calculations behind the chart types which hasn't always been the case, resulting in error. Also, with time becoming a limiting factor in many roles, the need to manually work out formulas for charts is deterring people from recording their data in this way.’

All in all, maybe the manual creation of SPC charts is not the way forward and it might be worthwhile looking at QI software systems that could calculate the charts for.

Let’s take a look at other ways you can create your control or SPC charts.

Creating SPC charts using templates such as Excel

Later on in this article, we’ll be looking more closely at specialist QI software that you can use to help you create all elements of your QI charts, and how specialist software has revolutionised the way people create Control or SPC charts. But there are other options you can look at. For example, you can create SPC charts using templates such as Excel, in fact NHS England provides one.

With this tool you can:

• ‘record your data on a daily, weekly or monthly basis and plot it on an SPC chart automatically
• indicate when a process may have changed by automatically applying SPC rules to the data
• annotate your chart and when appropriate apply step changes to your mean and process limits’

There’s another Excel template from ASQ that you could use to create your SPC chart.

However, it’s worth remembering that – as with many charts used in QI - there are certain rules that you will need to interpret your SPC charts. If you are using specialist QI software, such as Life QI, then these rules will be flagged up in the course of creating your chart, so you don’t need to remember them.

So, while using an Excel template would still be more useful than creating and managing the SPC chart manually, we would really recommend using a specialist online solution such as Life QI to do so, to make your life easier.

Creating an SPC chart using an online solution such as Life QI

Software for Quality Improvement has revolutionised the way in which people manage their QI projects. It also helps present visual information in the form of Run and Control Charts. Even the NHS recommend: ‘generally we use specialist software to create SPC charts, but charts can also be easily created using MS Excel.’

Software solutions like Life QI have been helping organisations to create SPC charts and manage their QI projects for years. When you use a solution such as Life QI, it brings together your data from multiple sources, which helps you to visualise and display your data and the results of your changes.

It’s really easy to create an SPC chart in Life QI. In our help center we have a very useful article – and have a helpful video on how to set up an SPC chart.

Let’s take a look at how you create an SPC chart in Life QI:

1. Firstly, you need to choose the measure you would like to create and SPC chart for.
2. Next, click on the 'Add a chart' button and give the data source a name.
3. Click on the 'Add row' button and add your data manually or, in case you would like to add more data points, copy and paste your data from you Excel spreadsheet; you can paste in as many complete rows of data as you want with one paste action.
4. Finally, click on 'Create chart'. You'll see the SPC chart automatically created for you and added to your project area.

Using Life QI for your SPC chart helps you to do the following:

• Auto-calculate the data line, centre line and control limits
• Auto-detect any Special Cause Variation in your data
• Annotate key events
• Annotate where PDSA cycles began

Having looked at three different ways of creating an SPC chart, I think you might agree that using a software solution such as Life QI to create an SPC chart really is as simple as that! If you want to find out more or see a demo, contact us.