Two logics, one diagram
- Left — tree logic descending from an event to its causes, flattened in a bowtie into straight lines.
- Right — branching logic after an event according to whether barriers work, reduced in a bowtie to consequence boxes.
What is lost
| Present in trees | In a bowtie |
|---|---|
| "And" / "or" gates | Absent — every threat is treated as an independent pathway |
| Probability arithmetic | Absent — no numeric result |
| Common cause analysis | Partly — the escalation factor column takes its place |
| Branch probabilities | Absent |
What is gained
Readability on one page. A bowtie can be argued over at the same table as the crew; trees are usually presented rather than argued over. The real strength of the method is not arithmetic but shared understanding: maintenance, operations and safety looking at one diagram talk about the same barrier.
When to go back to a tree
If two causes only produce the top event together, a bowtie cannot show it and draws them as two separate pathways — making the protection look weaker than it is. That pathway needs tree logic.
Order of use
The productive order is to start with the bowtie and deepen where needed. A bowtie shows cheaply which pathway deserves depth; trees resolve that pathway expensively.