Altangerel and Spaeth should be congratulated on their novel approach to patient-centred measurement of visual disability in glaucoma (The Assessment of Function Related to Vision [AFREV]).Citation1 Indeed, better measures of patient-centred performance in glaucoma have been called for.Citation2 Particularly pleasing is their use of Rasch analysis, as the importance of this technique in both providing truly linear scoring and an insight into questionnaire validity is gaining widespread appreciation in ophthalmology.Citation3 However, Rasch analysis is only useful if used correctly. Several aspects of the analysis performed by Altangerel and Spaeth deserve attention.
It is not really possible to assess the overall performance of the AFREV within the Rasch model because the authors have not reported any overall model statistics. Typically, statistics such as person and item separation reliabilities for the model and the real data are reported.Citation4 These give an indication of test precision which is dependent on having sufficient numbers of items.Citation5 It is very concerning that the final version contained only five items in the Rasch model because such a small number of items is unlikely to provide a valid model. Even including all nine original AFREV items may be cutting it fine, but this cannot be assessed without overall model statistics being presented. Then, only if one has valid overall model statistics do item fit statistics become relevant.
The authors do present item fit statistics and use them to decide whether to remove misfitting or overfitting items to improve item. While this is a standard approach, it should only be done within the confines of good overall model performance.Citation4 One would want to see evidence that item reduction has not led to an unacceptable loss of precision.
Targeting of items to patients is a key element of validity that the authors seem mindful of.Citation4 However, the tasks are very poorly targeted to the patients, with the overwhelming majority of patients not having difficulty with any of the tasks. This illustrates that the AFREV test would benefit from the addition of more difficult tasks.
These findings limit the claim that “AFREV appears to be a valid measure of performance ability.” Curiously, while the authors outlined the virtues of Rasch analysis, they also used the NEI-VFQ questionnaire and scored it with Likert scoring. Others have shown the benefits of Rasch scaling the NEI-VFQCitation6 (indeed a conversion algorithm was published in this journal),Citation7 so why not Rasch scale these data also?
The Author's Reply
We appreciate the opportunity to respond to Dr. Weisinger's letter regarding our contribution on “Assessment of Function Related to Vision (AFREV).” His comments are on target. Furthermore, they are gracious. His concerns are shared by us.
As a result of our experiences with AFREV, a second-generation study, and our at least partial understanding of its limitations, we have developed a third-generation instrument; we modified the subtests markedly, and we hope that in the new version, which we call the Assessment of Disability Related to Vision (ADREV), some of his concerns will be answered. For example, those items that appeared in AFREV not to be appropriate to consider using a RASCH analysis have been deleted from the third-generation test, and new items have been developed to assess specific aspects of visual functioning. Furthermore, the range of responses within each of the subtests has been widened, so that for all items the full spectrum of difficulty within the subtests is represented.
Regarding the NEI-VFQ questionnaire, we have concerns about this entire test. While several authors have described this as a “validated” questionnaire, we question the accuracy of the word “validated.” It appears that the test is, in some people, reproducible, but whether it is a valid, generalizable measure of quality of life is another issue. Our primary interest, as properly noted by Dr. Weisinger, is in a useful performance-based measure. Our comment that we believed that AFREV was a useful test relates to opinions that we still hold; specifically, that a method of assessing meaningfully how the various aspects of visual ability affect an individual's capacity to act in the real world is needed and that a comprehensive test involving different types of activities is more likely to achieve this goal than one assessing a more limited number of functions.
We are grateful for Dr. Weisinger's comments.
REFERENCES
- Altangerel U, Spaeth G L, Steinmann W C. Assessment of function related to vision (AFREV). Ophthalmic Epidemiol. 2006; 13(1)67–80
- Spaeth G, Walt J, Keener J. Evaluation of quality of life for patients with glaucoma. Am J Ophthalmol. 2006; 141: S3–14, (1 Suppl)
- Pesudovs K. Patient-centred measurement in ophthalmology—a paradigm shift. BMC Ophthalmol. 2006; 6(25)
- Pesudovs K, Garamendi E, Elliott D B. The Quality of Life Impact of Refractive Correction (QIRC) Questionnaire: development and validation. Optom Vis Sci. 2004; 81(10)769–777
- Mallinson T, Stelmack J, Velozo C. A comparison of the separation ratio and coefficient alpha in the creation of minimum item sets. Med Care. 2004; 42: I17–24, (1 Suppl)
- Massof R W, Fletcher D C. Evaluation of the NEI visual functioning questionnaire as an interval measure of visual ability in low vision. Vision Res. 2001; 41(3)397–413
- Massof R W. Application of stochastic measurement models to visual function rating scale questionnaires. Ophthalmic Epidemiol. 2005; 12(2)103–124