SC QUALITY GUILDInterrelationship digraph
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List the issues, then draw an arrow from each cause to its effect. The tool counts the arrows in and out of every issue, names the key driver and the key outcome, and draws the digraph.
ON THE EXAM · CQPA II.D.1, CQE V.B.5, CSSBB IV.D.7 · FREE · NOTHING YOU TYPE LEAVES YOUR BROWSER
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What the digraph is for
An interrelationship digraph (ID, also called a relations diagram) shows how a set of issues affect one another. It is used when a problem is complex and the issues are tangled together, and the team needs to know where to start. Instead of attacking every issue at once, it finds the few that drive the others.
The issues often come from the header cards of an affinity diagram. Five to ten issues is the workable range.
How to build one
- Arrange the issues, usually in a circle so that every pair can be connected.
- Take each pair in turn and ask: does one cause or influence the other? If not, draw nothing.
- If they are related, decide which direction is the stronger influence and draw a single arrow from the cause to the effect. The method allows one arrow per pair; two-way arrows hide the decision the team has to make.
- Count the arrows going out of and coming into each issue.
With n issues there are n(n − 1)/2 pairs to consider: 28 for eight issues. Going through all of them is the work; skipping pairs gives misleading counts.
Reading the counts
- Key driver: the issue with the most arrows going out. It is a root cause or a lever: improving it moves the issues downstream. Work here first.
- Key outcome: the issue with the most arrows coming in. It is a symptom, and a good measure of success, but attacking it directly rarely lasts.
- Issues with more out than in are mostly causes; more in than out, mostly effects. An issue with no arrows either does not belong or a link was missed.
Some practitioners also look at the total of in plus out: an issue with many arrows both ways is a bottleneck where many relationships meet.
Limits
The arrows record the team's judgment, not measured data. The digraph points to where to look; it does not prove cause. Confirm the key driver with data, for example with a cause and effect analysis and a test, before committing resources to it.
On the exam
Interrelationship digraphs are listed among the quality management tools in CQPA (II.D.1), the quality management and planning tools in CQE (V.B.5), and the analytical tools in CSSBB (IV.D.7). Expect to read a digraph, count the in and out arrows, and identify the key driver (most outgoing) and key outcome (most incoming).