Cumulative Flow Diagram
- Eloise Vinson
Cumulative Flow Diagrams (CFDs) depict overall process stability. This chart is the only flow analytic that simultaneously shows the relationship between the three metrics of flow mentioned in Little's Law: Work In Progress (WIP), Cycle Time, and Throughput.
What you'll find in this article
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How to read the chart
A cumulative flow diagram is a stacked area chart. The vertical axis represents the cumulative count of work items. The horizontal axis represents calendar days. Each colored band on a CFD represents one workflow stage defined for your data.
The height of a colored band (a workflow stage) or all the colored bands combined minus the last (the system) represents the total work in progress on a given point in the horizontal axis (or calendar day).
The width of a colored band (a workflow stage) or all the colored bands combined minus the last (the system) represents the approximate average cycle time of work finishing on the date where your measure hits the last colored band.
Please also see the summary statistics and rate lines sections of this document for more quick information you can get by looking at this chart.
The CFD gives clues that must be looked into
Looking at the colors to see how work flows and accumulates in your system generates lots of good questions to help uncover, not only how you use your workflow, but what issues lie within that keep you from delivering a steady flow of work. For instance:
If one color bulges, it means work is entering that workflow stage but not moving out as fast. You might ask: "What is causing work to pile up here?"
If one color disappears, it means that all work has departed that stage, and no new work has entered it. You might ask: "Why have we stopped doing this workflow step?"
The "ideal" is to have parallel lines for all stages going up and to the right.
Key Chart Controls
Summary Statistics
This summary stats box gives you the following quick stats, based on the data you have selected:
Arrival Rate (in items/day)
Throughput (in items/day)
Daily WIP (in items)
Cycle Time (in days)
This information is provided in a tabular format with rows representing each individual workflow stage but also one for the system (which is for your workflow as a whole). Please note that all values are calculated averages.
This panel updates as you filter or zoom in or out on your source data.
Rate Lines
These are an easy way to glean some of the most important information from the CFD at a glance. It can tell you just by the slope of the lines if your system is stable. By stable, we mean, is the rate at which work enters your system (the top rate line) about the same as the rate at which it exits your system (the bottom rate line).
Interpreting what you see
Parallel lines - Lines staying the same distance apart.
Parallel lines mean you are finishing roughly the same amount of items as you start (items enter the first and last checked workflow stages at the same rate). In other words, you are keeping pace with demand.
Diverging lines - Lines getting farther away as you move to the right.
Diverging lines mean that you are finishing fewer items than you start, roughly (items enter the first checked workflow stage at a faster rate than they enter the last checked workflow stage). In other words, your pace of finishing work is falling behind that of starting work.
Converging lines - Lines getting closer together as you move to the right.
Converging lines show that you are finishing more work than you start, roughly (items enter the first checked workflow stage at a slower rate than they enter the last checked workflow stage). In other words, your pace of finishing work is exceeding that of starting work.
Tooltip: Cycle Time and WIP
These tooltips let you see at a glance information for each stage or for the system as a whole.
The picture below shows you the:
Cycle Time: It tells you the calculated average cycle time for the system or a particular stage as measured on that day.
and WIP: It tells you the number of items in the system or a particular stage on that day.
Workflow Stages
What happens when you deselect a workflow stage depends on where the stage is located.
If you deselect workflow stage(s) at either end of the workflow, you are tracking the flow of work through a smaller portion of your overall workflow.
This is a common usage of the Workflow Stages chart control.
If you deselect workflow stage(s) but have checked stages either side, the overall rate lines don't change because the same number of items still entered and exited your workflow on the same dates. However, you have removed a part of the workflow. So, in order to handle this unexpected loss of data, the chart adds the data for the unchecked stage(s) to the preceding checked stage.
All workflow stages are visible in the left-most image above. The data for the yellow workflow stage shows 1 day and 3 items.
In the right-most image above, you see what happens to the data when we deselect the yellow workflow stage in the middle of the workflow. The data for this stage was added to the turquoise workflow stage which reads 1 day and 10 items when previously the number of items was 7. The 3 from the yellow was added to it to make 10.
This is not a common usage of the Workflow Stages control.
Would you like to examine a specific section of your chart more closely? You can now zoom in without affecting any of the calculations activated when modifying the selected data for the chart. These small enhancements can greatly improve your analysis of complex data sets!
Additional Chart Controls
Date Range
The Date Range control allows you to select a specific date range for your completed items across all charts.
Selecting the dates opens a calendar window where you can choose the date range that you want to see.
There are presets to select from, such as Last 30 Days, Last Quarter, etc.
CFD Layers
"Smooth" CFDs are really approximations of what happens between data points. To see a more realistic view of what is happening with your process data, uncheck the checkbox labeled: Smooth / Interpolate.
The button to toggle the Legend is here as well. The legend is on by default, but if you close it, you can come back here to show it again.
Layout
Check/Uncheck the item labeled "Show Date Control" to toggle the source data on and off. When it is on, you are able to see what data is being used for calculating and rendering the chart. You are also able to zoom in on selected data and also use the Selected Dates option to input the exact dates you want to measure from. See the video on this page for a demo.
Item Filter
You can filter down the items used to build this chart by choosing one or more available filters. If you want to clear your filters so that all work is represented again, you click the Reset button.
If you have any questions, please submit them to our help desk.
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