In this series of articles, we have been focusing on Statistical Process Control methodology. We’ve also had a look at control or SPC charts – a really useful way of plotting and measuring data. However, there are other charts you can use - such as run charts - in your Quality Improvement (QI) project.
So, in this piece we are going to focus on run charts vs control charts and how you might go about choosing which one to use in your improvement journey.
What’s the difference between a Run Chart and A Control Chart?
You may well have come across both run charts and control charts in your QI journey so far. Run charts and control charts are both important and valid QI tools, but have very different analytical and reporting abilities. Let’s take a closer look at both.
Run charts are one of the simplest charts to use in Quality Improvement, but they are still able to provide really useful information that can help guide you in your QI programme.
Run charts provide a ‘line chart of data’ which is plotted over time. This offers a straightforward way of helping you to discover patterns and trends within your projects.
As you continue to plot the data, this visualisation of data will help you to identify progress and patterns. Its simple nature means that you will be able to easily analyse and monitor your results.
The NHS Improvement document ‘An Overview of Statistical Process Control (SPC)’ says that run charts have traditionally been used ‘in service improvement to measure changes in a process over time.’
So far, so good! However, run charts do have limitations. There are a few things to note about them which sets them apart from other more sophisticated charts, such as SPC or control charts.
By way of an example, run charts do not have control limits. As run charts don’t display statistical control limits, they aren’t able to detect unusual levels of variation. Something that a SPC or control chart can do.
- Run charts have an X and a Y axis. Time is always shown on the horizontal axis, and the measure of improvement is shown on the vertical, Y axis
- Run charts have a median – used as a centreline
- Run charts are less sensitive than SPC charts
- Run charts can identify Common Cause and some types of Special Cause Variation
- Run charts can help when you start improvement initiatives, as you can start with a small amount of data.
Let’s take a look at control charts now.
Control or SPC charts
You could describe an SPC or control chart as a more advanced version of a run chart. Like run charts, they can identify different types of variation, but have different rules. Let’s find out more.
In addition to the mean or average, what is most noticeably different about SPC charts from run charts is their ‘control limits’. These are two extra lines that are calculated using modified statistics and which enable the user to understand natural variation.
The upper line represents the upper control limit and a lower line represents the lower control limit. The upper and lower control limits are boundaries which help show variation in the QI process.
Indeed, they let you identify the difference between normal and unusual variation in data and also whether the variation is positive or negative. You can read more about control limits in our next article.
For a control chart you do need more data –a minimum of 15 data points, and preferably around 20. This is because the ‘mean’ in your control chart is more sensitive than the median in a run chart to point-to-point variation.
- Control charts are more sensitive than run charts and so able to give more detail
- Control charts have a ‘mean’ or average which is plotted on the central line (CL)
- Control charts have control limits
- Control charts can define ‘process capability’
- Control charts are able to detect special causes and help the user more accurately predict what will happen
- You need more data for a control chart
If you want to find out more, the Institute for Healthcare Improvement (IHI) have a great video here. You can also read more in the NHS Improvement document ‘An Overview of Statistical Process Control (SPC)’.
How to choose the right chart
As we can see - there are significant differences between run charts and control charts or SPC charts. It would appear at first glance that run charts are easier to use and create, whereas control charts or SPC charts could potentially be off-putting due to the amount of statistical knowledge required to create and monitor them.
However, don't let the apparent complexity of control charts put you off using them! It’s really important to use control charts, as the results are so beneficial. With QI software solutions such as Life QI being available, you can use these to really help you simplify the whole SPC or control chart experience.
As control charts are ‘decision-making tools that provide information for timely decisions concerning recently produced products’ they are able to give you and your QI team so much more detail on trends and variation when you monitor and review your QI projects.
They give a greater depth of statistical interpretation, as they can show individual data points. As the IHI says: ‘For improvement efforts, visual displays of data are often the best approach to learn from variation in data. Images are usually easy and quick to prepare, and they make it possible to access nearly all kinds of potential insight from the data.’ SPC or control charts give you so much more visually.
So basically, the type of chart you choose is dependent the level of detail you want to understand on your project.
If you decide to go down the control or SPC chart route – you might want to take a look at this video from our help center. This video shows you how easy the process can be, when you are using software such as Life QI.
We can help you pick the right type of SPC chart - as it can be tricky working out which type of control chart to choose. Take a look at our 'SPC Chart Type' infographic which will guide you through the decision-making process and help you make the right choice for your project.
So, to conclude - even if using run charts is more simple in the short term, using a software solution such as Life QI will make creating an SPC or control chart much easier, and give you much better and more granular results on your QI project.