Table 5 Threats to validity
Threat to validityResponse/observation
Temporal effectsNone identified with positive bias
Observer bias (NOTECHS)Independent (uninvolved) third observer
Observer bias (OTEs/NOPEs)Objective, closely defined scales
Case selectionConsecutive series with specified exclusions
Selection bias in staff trainingAll relevant staff invited by same methods
Validity of scoring systemsValidation work (NOTECHS): previously validated scale (OCHRA)
  • NOPE, non-operative procedural error; NOTECH, Non-Technical Skills scale; OTE, operative technical error.