What is recalculating
When the process is known to change, for example by the change is indicated by a run test, the control limits should be recalculated. This is because when using the data from two different processes can yield incorrect results. However, using the same chart to visualize the data is an effect mean to communicate the size and direction of the change.
This is when the recalcution, or stepping is needed. Is short, it is used to indicate a change in process.
How it is done
Craydec Control Charts enable you recalculate the limits in control charts. The number of changes contained in single control charts is not limited.
The change point is indicated by data role ‘Step / Subgroup’. When visual finds change in the data role’s value, it will recalculate until the next change or end of data.
How to indicate a change using data
To make the most of the following text, download our sample report. Note that you need a valid license key to enable the full features of the visual.
1) Column in a data table
Easiest way to use stepping is add the required data role to the actual data table. This done in the example report using the table ‘ImR’, and the column ‘subgroup’ (ImR[subgroup]).
The stepping is done by preparing a column which indicates the data points belonging to in single step, and this column is added to the visual using the data role ‘Step / Subgroup’.
This is good option as it easy and fast understand, even when you maintaining a report someone else has made. It’s also a good option when the data is prepared in some other system or staging area, as it is then easy to add the required data to the table.
However, when manually updating the data this option may be too time consuming.
2) Change points lookup table
Setting up a look up table requires a bit more work, but maintaining the table is fast and easy. It also enables to recalculate the limits without republishing the whole report, you only need to refresh the data.
Lookup table contains two columns. First, a link column (unique_id in the example report). This column is used identify from which data point the change starts. The second column is the name of the group (step in the example report). The name can be almost anything as long as the value is different from the previous value. Numbers and texts are good names for the step.
1 - create a lookup table
The lookup table can be imported from any data source, depending where you want to maintain the data. The easiest method to create it, is using the PowerBI’s ‘enter data’ function, without need to import a table. Hint: the data can be update using the ‘Edit queries’ function and selecting the ‘source’ step.
2 - create calculated column to data table
After lookup table is created, add a calculated column to the data table (not in the lookup table). See the formula from example report, from column ImR[step].
Note: when copying the formula, you must change the table and column names to match the ones you use.
3 - enter the change points
Last thing to do, is to enter the change points to the lookup table. You’ll only need to insert the points where process has changed.