| Threat to validity | Response/observation |
| Temporal effects | None identified with positive bias |
| Observer bias (NOTECHS) | Independent (uninvolved) third observer |
| Observer bias (OTEs/NOPEs) | Objective, closely defined scales |
| Case selection | Consecutive series with specified exclusions |
| Selection bias in staff training | All relevant staff invited by same methods |
| Validity of scoring systems | Validation work (NOTECHS): previously validated scale (OCHRA) |
NOPE, non-operative procedural error; NOTECH, Non-Technical Skills scale; OTE, operative technical error.