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71 Gener a l info r ma tio n
E l e k t r o n E y e T e c h n o l o g y 2 0 1 8
4. 3
Ca tch tr ia ls
Single stimulus tests incorporate catch trials to give the clinician an estimate of the patient's
reliability.
False positives:
when no stimulus is presented.
False negatives ( only in threshold and D riving tests) :
when a repeat
presentation is made at an already seen location.
Catch trials occur at random intervals and the results are given, as a fraction and percentage,
on the status bar at the bottom of the test screen.
The faction numerator is the number of errors and the denominator the number of catch trials.
For example, 3/10 false positives would indicate that the test had made 10 false positive trials
and the patient had, in error, responded to 3 of them.
Normally the number of false positives should be less than 15% and the number of false
negatives less than 30%. False negatives increase if there is a visual field defect.
This is due to the increased variability of the patient in areas of reduced sensitivity.
The relationship between catch trial responses and test-retest variability is biased towards the
patients reliability and the results from these trials should only act as a guide.