Three dimensions
| Dimension | Question | Evidence |
|---|---|---|
| Effectiveness | Does it stop the pathway on demand? | Test result, design calculation |
| Availability | Is it ready when needed, or bypassed? | Record of time spent out of service |
| Reliability | Does it repeat the same performance? | Failure history, test frequency |
Why there is no number
A bowtie is qualitative: it has no scale table, no formula and no result band. That is a design choice rather than a gap — the method exists to make barriers visible, not to produce a score.
Being able to say NOT MEASURED
We could not find numeric thresholds for these three dimensions in the source we measured, and we write that line here plainly. It would have been easy to write "availability should exceed 95 per cent"; presenting an unmeasured number as measured is the thing these pages do not do.
If a number is needed
Where a numeric result is required, the method changes: layer of protection analysis works with demand frequencies and probabilities of failure on demand, and its bands are defined. Using the two methods side by side is the right answer, rather than forcing numbers out of a bowtie.
Qualitative is not empty
The absence of a number does not make the assessment arbitrary. Each dimension demands evidence: a test record, out-of-service time, a failure history. A "good" rating with no evidence behind it is not an assessment.