Table 2. Assessment of HRQoL measures.

Reliability	• ‘Ability of a measure to reproduce the same value on two separate occasions when there has been no change in health'39 • Can be over time or between methods of administration39 • May be considered in terms of internal consistency (the extent to which all items measure the same concept or test–retest reliability (the extent to which the results of the instrument compare if the test is administered to the same subject on more than one occasion when there has been no demonstrable change in health status)
Validity	• The extent to which a measure reflects the concept that it is intended to measure • May be considered in terms of content validity (‘degree to which the instrument is reflective of aspects important to the patients and disease of interest'), construct validity (‘how well a measure correlates with other indicators of similar or related constructs'), concurrent validity (‘the extent to which an instrument correlates with other measures of the same or similar construct'), and discriminant validity (‘the ability to discriminate between either cases vs controls or disease severity groups')40 • For the purpose of this paper, construct validity will be determined if compared with objective clinical measures such as visual acuity; concurrent validity will be a comparison to an existing vision-specific HRQoL measure • Factor analysis is a method of determining the structure of an instrument in terms of domains or subscales. It can be used to identify redundant or duplicate items. It may also be used to determine the domain structure. Some papers refer to this as a measure of internal validity
Responsiveness	• The extent to which the instrument can detect in patients known to have a change in their physical condition