What is an interrelationship diagram?
A technique for organizing a problem where causes and effects are too tangled to fit into a one-directional tree, by treating cards as a freely connected network of arrows. It’s one of the tools grouped under the “New QC Seven Tools” used in quality management.
Logic trees and fishbone diagrams assume a one-directional breakdown — top to bottom, cause to effect. An interrelationship diagram allows for things like:
- Causes influencing each other (A worsens B, and B in turn worsens A)
- Arrows looping back around into a vicious cycle
The messier a problem refuses to fit into a clean hierarchy, the more this technique is called for.
Where this works well
- When you tried writing out causes and found they don’t break down cleanly into a tree — they influence each other instead
- When you want to visualize a problem that feels like it’s “just getting worse” in a vicious cycle
- When you want to identify, before taking action, which point would have the biggest impact on the whole system
How to build it in ThinkTray
Step 1: Write the central issue on a card
Use the Card tool (R) to write the issue you want to focus on at the center.
- Example: “New staff take too long to get up to speed”
Step 2: Write out likely factors as cards around it
Place cards for whatever factors come to mind around the center, without judging them.
- Don’t worry about position at this stage — you can move things later
Step 3: Connect “cause → effect” relationships with arrows
Use the Line tool (L) to draw an arrow from whichever side has the influence to whichever side is affected.
- Unlike a tree, it’s fine for multiple arrows to leave one card, or multiple arrows to arrive at one card
- Connect anything you can confidently say “A, therefore B” about
Step 4: Look for arrows that loop back (vicious cycles)
Once everything is connected, trace the arrows and check whether any of them loop back to where they started.
- If you find a loop, that’s where a vicious cycle is happening. Make it stand out — a different line color, for instance
- The demo above highlights the vicious-cycle loop with a red dashed arrow
Step 5: Pay attention to cards with many outgoing arrows (advanced)
A card with more outgoing arrows (more influence on other things) is more likely to be a root cause.
- Highlighting those cards with a thicker border or a distinct color makes it easier to weigh priorities for taking action
Interrelationship diagram vs. logic tree vs. fishbone diagram
| Technique | Structure | Good fit |
|---|---|---|
| Logic tree | Top-down hierarchy | When causes break down cleanly |
| Fishbone diagram | Spine + category bones | When you want to surface causes exhaustively by fixed category |
| Interrelationship diagram | Free network (loops allowed) | When causes influence each other and won’t fit one direction |
Starting with a logic tree or fishbone diagram and switching to an interrelationship diagram the moment it “just won’t turn into a clean one-directional tree” tends to go smoothly.
Tips
- Don’t try to connect every possible pair — Only connect relationships you can confidently call “A, therefore B.” Forcing connections makes the diagram harder to read.
- Set aside cards you’re unsure about the direction of — If something feels like it influences both ways, try drawing two arrows and revisit it later.
- Once you find a loop, look for one point that breaks it — A vicious cycle can often be stopped by changing just one link in the chain.
- Tidy up the layout periodically — Once arrows start crossing and it’s hard to read, move cards around to make it clearer again.
Interrelationship diagrams are grouped together with several related techniques under the “New QC Seven Tools,” but accounts of how that set was formalized vary between sources, so this article doesn’t attribute it to a specific inventor or organization.