‘Overlap zone’ | The range of visions within which there is uncertainty whether a participant will pass or fail the number-plate test (see figure 1). The minimum is the best vision with which a participant failed the number-plate test, and the maximum is the worst vision with which a participant passed. To allow parity in the comparison of different charts, only those participants with vision within the overlap zone are considered when calculating the remaining parameters |
False positive rate | Indicates the proportion of participants within the overlap zone who did not achieve 6/12, yet were able to read a number-plate. Such participants would have satisfied the previous regulations (number-plate only), but do not satisfy the revised regulations |
False negative rate | Indicates the proportion of participants within the overlap zone who achieve 6/12 acuity, but were unable to read a number-plate. Such participants could be incorrectly told their vision was good enough to drive based on their vision in the clinic |
Sensitivity | Represents the ability of the visual acuity cut-off to predict failure in the number-plate test. It is calculated by the expression TP/TP+FN, where TP is the number of ‘true positives’, or a fail on both criteria, and FN is the number of false negatives13 |
Specificity | Represents the ability of the visual acuity cut-off to predict a pass in the number-plate test. It is calculated by the expression TN/TN+FP, where TN is the number of ‘true negatives’, or a pass on both criteria, and FP is the number of false positives13 |