One of the major deliverables coming out of define and measure phases is the baseline metric. How is the process performing now, and what measurement will the team use to compare current performance to post-improvement performance?
Baseline metrics are numbers, but most teams find that presenting the metric graphically resonates best with business resources and executives. Visual representations also provide teams with a quick way to determine if progress is occurring.
The type of visual representation you use depends on whether your major metric is discrete or continuous. Discrete data can be displayed on Pareto charts and continuous data can be displayed via run charts. You can also use variation or other calculations to convert discrete data to continuous data for display in run charts and control charts.
What is Run Chart?
A run chart can be used to monitor the performance of any variable or process over time. With a single, intuitive chart, Six Sigma teams can display trends, shifts, and cycles within a process; they can also monitor a process for concerns, though run charts are not as effective at this as the very similar control chart is.
A basic run chart is simply a line plot of the data over time, which means anyone can create the chart. Most Six Sigma run charts also feature a line representing the median of all data points for visual reference. Depending on the type of information being charted, you may need to convert data to a ratio for a more accurate run chart. For example, if you are plotting the temperature of a surface over time, there is no need to convert data. If you are plotting the number of patients readmitted to a hospital within 30 days of being discharged, then it helps to convert the data to a percentage of the number of patients discharged within the same time period. In a 30-day period where 10,000 patients were discharged, you can expect a higher number of returns than a period during which only 5,000 patients are discharged.
The figure below illustrates a run chart of temperature over time. You can see how temperature changes through time and begin to see some possible trends. A Six Sigma team would be able to zoom out, viewing the run chart over more time to validate trend assumptions. You can also see that the median temperature for the process is 33.
The run chart below indicates the number of returns per hundred sales for each month of the year. You can see that returns as a rate of sales increases steadily during the first part of the year before holding steady from May through November. The orange line indicates the median returns per hundreds sales, which is just under 7.
How to Create Basic Run Chart in Excel
Statistical analysis software, including Minitab and Excel SPC, creates all elements of a run chart automatically from entered data, but anyone can use basic Excel functions to create a run chart if needed.
First, create a data table.
Creating a data table for a single attribute, such as temperature, just requires entering the time labels in one column and the attribute measurements in another. For the example, we’ll walk through creating a rate data table, since it involves additional steps.
- Enter the data labels (month, week, hour, etc.) in the first column of Excel.
- In the next column, enter the corresponding measurements for the attribute you are interested in: in this case, the total number of returns per month.
- In the third column, enter the total number of items you are comparing the attribute to: in this case, the total number of sales per month
- In the fourth column, calculate the percentage the first column of data is of the second. In this case, the percentage of returns per sales for each month. The calculation is achieved in this case by the formula =B2/C2 for January, =B3/C3 for February, and so forth.
- Decide whether you want to create a run chart showing percentages, or if you would like to create the chart illustrating rate per 100, per 1,000, etc.
- If you want to illustrate a rate per (x), multiply the percentage calculation in the fourth column by (x). In this case, the figures in column D are multiplied by 100.
- Use Excel to calculate the median of the number you plan to chart. The median is calculated with the formula =Median(Number 1, Number 2,…), where the numbers in the formula correlate with the range of all the charted data points. In this case, the median is 6.96679
- Highlight the data labels (in this case, column A) and the figures to be charted (column E)
- Select Insert > Charts > Line Chart to insert a line chart of the attribute or attribute calculation.
- Select Insert > Shape > Draw Line.
- Draw a line on your run chart approximately where the median would be. Use Excel tools to select a color and thickness for the line that you desire.
The completed run chart can be used to present information to the Six Sigma team or include a graphical representation of baseline process performance in a measure phase tollgate presentation.
Again, it should be noted that manual creation of a run chart is not required for most Black Belts and Green Belts, who will have access to statistical analysis software.