Clarification regarding 'technical failure'
I appreciate Trevor Warburton's comments and am in broad agreement with his views. He presents an excellent summary of the optometrists perspective and highlights many important issues.
About the issue of 'technical failure'; it can be interpreted as a failure of the screening system or a failure of the particular modality (ie a equipment failure). To enable comparison between different modalities of screening, we have assumed that a 'technical failure' is a failure of the particular modality of screening. The latter definition, whilst equally acceptable, cannot be used as the basis of comparison between two systems which use different equipment-sets.
Competing interests: No competing interests