The Refractive Status and Vision Profile: Rasch analysis of subscale validity

The Refractive Status and Vision Profile: Rasch analysis of subscale

validity

Konrad Pesudovs, Vijaya Gothwal, Thomas Wright, David Elliott

NH&MRC Centre for Clinical Eye ResearchFlinders Medical Centre & Flinders University

Adelaide, South Australia, Australia

Financial Disclosure: The authors have no financial interest

• It is important to measure the outcome of refractive surgery from the patient’s point of view

• Several questionnaires exist for this purpose• One, the Refractive Status and Vision Profile (RSVP),

contains 8 subscales which suggest the measurement of many aspects of quality of life (QOL)

• However, the validity of subscales cannot be inferred from overall validity of a questionnaire and the subscales of the RSVP have never been thoroughly examined with modern psychometric methods

• Purpose – to examine the validity of the 8 subscales of the RSVP questionnaire using Rasch analysis

Background and Purpose

Patients and Methods

• 200 myopic patients recruited from a refractive surgery setting and general optometric practice in the UK

• Mean age, 38.5 yrs

• 61% females

• 47.5% contact lens wearers

• Spherical equivalent -4.25D 2.50D

• Completed the RSVP by self-administration in clinic

• The RSVP comprises 42 questions (items) scored in eight subscales – Concern (6 items)– Expectations (2 items)– Physical/social functioning

(11 items)– Driving (3 items)– Symptoms (5 items)– Glare (3 items)– Optical problems (5 items)– Problems with corrective

lenses (7 items)

• Rasch analysis was performed using Winsteps software (version 3.68)

• This is an item response theory model which estimates questionnaire scores on a linear scale and gives in-depth insight into aspects of validity

• Key aspects of validity:– Precision of measurement (can the instrument discriminate

between people, denoted by the Person Separation statistic: minimum acceptable value 2.0)

– Unidimensionality (do all items in a subscale measure a single concept)

– Targeting of items to people (do the items suit the people being measured (or are they too easy/hard); denoted by the difference in person and item mean value in logits)

Rasch Analysis

• Six subscales had inadequate person separation (<2.0), chiefly due to having too few items

• Only 2 subscales had adequate person separation– Concern and Driving

• Both of these subscales were unidimensional• However, both poorly targeted patient QOL

– Driving was mis-targetted by 7.02 logits indicating almost no patients had any difficulty with driving

– Concern was mis-targetted by 1.11 logits indicating patient’s QOL was better than concern addressed by the items

Results - RSVP subscales

Conclusions

• None of the subscales of the RSVP are valid

• The RSVP should be considered as a single measure of refractive error related quality of life without subscales

• Better questionnaires exist for the measurement of patient reported outcomes of refractive surgery (e.g.QIRC1)

1. Pesudovs K, Garamendi E, Elliott DB. The Quality of Life Impact of Refractive Correction (QIRC) questionnaire – development and validation. Optom Vis Sci 2004;81:769-77.