We would like to thank Dr. Pesudovs for his interest in our recent publication on the development of a symptom questionnaire for virtual reality viewing.1
There is a range of issues associated with the use of descriptors on questionnaire rating scales. For example, if just the end categories are labelled (i.e. end-anchoring) responses can be drawn towards the ends, whereas if every other category is labelled then the labelled categories are more likely to be endorsed.2 However, providing descriptors is also thought to make the task less abstract.3 This was the reason for their inclusion in the VRSQ. Regardless of the number of descriptors used on a questionnaire scale there exists the problem that in the assignment of ordinal values to categories, the interval between categories cannot be assumed to be equal. Thus the application of non-parametric statistics is necessary for the analysis of questionnaire data. Although Rasch analysis can be useful in attempting to linearise an ordinal scale it was not appropriate to apply to the data presented in our manuscript on the VRSQ as the subject numbers were not sufficiently large. A minimum of 200 subjects is considered necessary by Streiner and Norman,2 although Pesudovs et al.4 and Gothwal et al. 5 have applied it to the data from 43 and 78 subjects respectively. Nonetheless, we agree with Dr Pesudovs in that it would be informative to apply Rasch analysis to the VRSQ when tested on a larger subject group to further validate it as a useful tool.
Shelly L. Ames
James S. Wolffsohn
Neville A. McBrien
Department of Optometry and Vision Sciences, University of Melbourne, Melbourne, Victoria, Australia
1. Ames SL, Wolffsohn JS, McBrien NA. The development of a symptom questionnairefor assessing virtual reality viewing using a head-mounted display. Optom Vis Sci. 2005;82:168–76.
2. Streiner DL, Norman GR. Health Measurement Scales: A Practical Guide to their Development and Use, 2nd ed. Oxford: Oxford University Press, 1995.
3. Dixon PN, Bobo M, Stevick RA. Response differences and preferences for all-category- defined and end-defined Likert formats. Educ Psychol Meas. 1984;44:61–6.
4. Pesudovs K, Garamendi E, Keeves JP, Elliott DB. The Activities of Daily Vision Scale for cataract surgery outcomes: re-evaluating validity with Rasch analysis. Invest Ophthalmol Vis Sci 2003;44: 2892–9.
5. Gothwal VK, Lovie-Kitchin JE, Nutheti R. The development of the LV Prasad- Functional Vision Questionnaire: a measure of functional vision performance of visually impaired children. Invest Ophthalmol Vis Sci 2003;44:4131–9.