Jyoti Khadka

Statistical methods for conducting agreement (comparison of clinical tests) and precision (repeatability or reproducibility) studies in optometry and ophthalmology

Citation information: McAlinden C, Khadka J & Pesudovs K. Statistical methods for conducting agreement (comparison of clinical tests) and precision (repeatability or reproducibility) studies in optometry and ophthalmology. Ophthalmic Physiol Opt 2011, 31, 330–338. doi: 10.1111/j.1475‐1313.2011.00851.x

A Comprehensive Evaluation of the Precision (Repeatability and Reproducibility) of the Oculus Pentacam HR

PURPOSE To evaluate the precision (repeatability and reproducibility) of the Pentacam HR (high-resolution) tomographer (Oculus, Wetzlar, Germany) across a large range of measurement parameters. METHODS A randomly selected healthy eye of 100 subjects was scanned twice with the Pentacam HR by one observer for each of the three measurement modes: 25-picture (1 second) scan, 50-picture (2 second) scan, and cornea fine scan (50 pictures in 1 second). The repeatability of each scan mode was assessed. One additional 25-picture scan was acquired by a second observer to test reproducibility. RESULTS Overall, the Pentacam HR had good precision, with the cornea fine scan returning the most precise results: The 25- and 50-picture scans showed similar precision. The repeatability limits, expressed as the within-subject SD × 1.96√2 of the anterior keratometry (K)1 and K2 readings with the standard 25-picture scan, were 0.25 and 0.36 D, respectively. Pachymetry maps, corneal maps, anterior chamber depth maps, corneal volume, topometric Q values and indices were also found to be precise. Poor precision was found for estimates of axis (astigmatic and progression index), pupil center pachymetry, single points on corneal maps, refractive power maps, and equivalent K readings. CONCLUSIONS Measurements taken with the Pentacam HR are repeatable and reproducible, especially those obtained with the cornea fine scan. Although the Pentacam HR is clearly a very useful clinical and research tool, the measurement of corneal axes, pupil center pachymetry, front meridional and axial maps, refractive power maps, and equivalent K readings should be interpreted with caution.

Quality Assessment of Ophthalmic Questionnaires: Review and Recommendations

Purpose The aim of this article was to systematically review all the available ophthalmic patient-reported outcome (PRO) instruments (questionnaires) that demonstrated interval measurement properties to identify the instruments with the highest psychometric quality for use in different eye diseases and conditions. Methods An extensive literature review was carried out to identify all existing ophthalmic PRO instruments. Instruments were then excluded if they did not have demonstrable interval measurement properties; the remaining instruments were reviewed. The quality of the following psychometric properties was assessed: content development (initial item development process), performance of the response scale, dimensionality (whether the instrument measures a single construct), measurement precision, validity (convergent, concurrent, discriminant, and known groups), reliability (test-retest), targeting (whether the items are appropriate [e.g., difficulty level] for the population), differential item functioning (whether subgroups of people respond differently to an item), and responsiveness. Results The search identified 48 PRO instruments that demonstrated interval measurement properties, and these were relevant to nine applications: glaucoma, dry eye, refractive errors, cataract, amblyopia and strabismus, macular diseases, adult low vision, children low vision, and others. These instruments were evaluated against the psychometric property quality criteria and were rated for quality based on the number of criteria met. Conclusions This review provides a descriptive catalog of ophthalmic PRO instruments to inform researchers and clinicians on the choice of the highest-quality PRO instrument suitable for their purpose.

Development of the 25-item Cardiff Visual Ability Questionnaire for Children (CVAQC)

Aims To develop and validate a short questionnaire to assess self-reported visual ability in children and young people with a visual impairment. Methods A list of 121 items was generated from 13 focus groups with children and young people with and without a visual impairment. A long 89-item questionnaire was piloted with 45 visually impaired children and young people using face-to-face interviews. Rasch analysis was used to analyse the response category function and to facilitate item removal ensuring a valid unidimensional scale. The validity and reliability of the short questionnaire were assessed on a group of 109 visually impaired children (58.7% boys; median age 13 years) using Rasch analysis and intraclass correlation coefficient (ICC). Results The final 25-item questionnaire has good validity and reliability as demonstrated by a person separation index of 2.28 and reliability coefficient of 0.84. The items are well targeted to the subjects with a mean difference of −0.40 logit between item and person means, and an ICC of 0.89 demonstrates good temporal stability. Conclusion The Cardiff Visual Ability Questionnaire for Children (CVAQC) is a short, psychometrically robust and a self-reported instrument that works to form a unidimensional scale for the assessment of the visual ability in children and young people with a visual impairment.

A Head-to-Head Comparison of 16 Cataract Surgery Outcome Questionnaires

PURPOSE To investigate the responsiveness of 16 questionnaires used in cataract surgery outcomes. DESIGN Prospective, observational study. PARTICIPANTS Patients at the Ophthalmology Eye Clinic, Flinders Medical Centre, Adelaide, Australia, and 1 matched eye clinic in Sweden. METHODS Sixteen Rasch-scaled cataract surgery questionnaires were completed before and 6 months after surgery. These were: the Cataract Symptom Scale, 6 versions of the National Eye Institute Visual Function Questionnaire, the Quality of Life and Vision Function Questionnaire, the Cataract TyPE Specification, the Visual Activities Questionnaire, the Visual Disability Assessment (VDA), the Visual Function and Quality of Life questionnaire, the Visual Function Index, Catquest-9SF, the Visual Symptoms and Quality of Life questionnaire, and the Cataract Outcomes Questionnaire. Responsiveness was calculated with the effect size (ES) statistic (change in questionnaire score divided by pooled standard deviation of the preoperative and postoperative scores). MAIN OUTCOME MEASURES Questionnaire responsiveness to cataract surgery (ability to detect clinically important change). RESULTS All 16 questionnaires and their subscales were responsive to cataract surgery, with visual functioning scales being more responsive than socioemotional scales and some subscales being less responsive. The largest ES was for the Catquest-9SF (1.45; 95% confidence interval [CI], 1.22-1.67), which was the only instrument with a mean and 95% CI of more than 1.0 (very large ES). Three measures had very large ESs and 95% CIs of more than 0.80 (large ES): the VDA (activity limitations and subscale) and the Cataract Outcomes Questionnaire, although their 95% CIs overlapped with a number of other instruments. CONCLUSIONS The Catquest-9SF is short and highly responsive to cataract surgery, and so is ideal for measuring visual functioning outcomes. Other instruments may be preferred to measure different constructs. FINANCIAL DISCLOSURE(S) The author(s) have no proprietary or commercial interest in any materials discussed in this article.

The importance of rating scales in measuring patient-reported outcomes.

BackgroundA critical component that influences the measurement properties of a patient-reported outcome (PRO) instrument is the rating scale. Yet, there is a lack of general consensus regarding optimal rating scale format, including aspects of question structure, the number and the labels of response categories. This study aims to explore the characteristics of rating scales that function well and those that do not, and thereby develop guidelines for formulating rating scales.MethodsSeventeen existing PROs designed to measure vision-related quality of life dimensions were mailed for self-administration, in sets of 10, to patients who were on a waiting list for cataract extraction. These PROs included questions with ratings of difficulty, frequency, severity, and global ratings. Using Rasch analysis, performance of rating scales were assessed by examining hierarchical ordering (indicating categories are distinct from each other and follow a logical transition from lower to higher value), evenness (indicating relative utilization of categories), and range (indicating coverage of the attribute by the rating scale).ResultsThe rating scales with complicated question format, a large number of response categories, or unlabelled categories, tended to be dysfunctional. Rating scales with five or fewer response categories tended to be functional. Most of the rating scales measuring difficulty performed well. The rating scales measuring frequency and severity demonstrated hierarchical ordering but the categories lacked even utilization.ConclusionDevelopers of PRO instruments should use a simple question format, fewer (four to five) and labelled response categories.

Reengineering the Glaucoma Quality of Life-15 Questionnaire with Rasch Analysis

PURPOSE To investigate, using Rasch analysis, whether the 15-item Glaucoma Quality of Life-15 (GQL-15) forms a valid scale and to optimize its psychometric properties. METHODS One hundred eighteen glaucoma patients (mean age, 65.7 years) completed the German-version of the GQL-15. Rasch analysis was performed to assess category function (how respondents differentiated between the response options), measurement precision (discriminative ability), unidimensionality (whether items measure a single construct), targeting (whether items are of appropriate difficulty for the sample), and differential item functioning (whether comparable subgroups respond differently to an individual item). Where any of these attributes were outside acceptable ranges, steps were taken to improve the instrument. RESULTS The five-response categories of the GQL-15 were well differentiated by respondents, as demonstrated by ordered and well-spaced category thresholds. The GQL-15 had an excellent measurement precision but demonstrated poor targeting of item difficulty to person ability and multidimensionality, indicating that it was measuring more than one construct. Removal of six misfitting items created a nine-item unidimensional instrument with good measurement precision and no differential item functioning but poor targeting. A new name, the Glaucoma Activity Limitation (GAL-9) questionnaire, is proposed for the short version, which better reflects the construct under measurement. CONCLUSIONS The GAL-9 has superior psychometric properties over the GQL-15. Its only limitation is poor targeting of item difficulty to person ability, which is an inevitable attribute of a vision-related activity limitation instrument for glaucoma patients, most of whom have only peripheral visual field defects and little difficulty with daily activities.

Improvements in visual ability with first-eye, second-eye, and bilateral cataract surgery measured with the Visual Symptoms and Quality of Life Questionnaire

PURPOSE: To determine whether the Visual Symptoms and Quality of Life Questionnaire (VSQ), which was developed to be sensitive to second‐eye cataract surgery, has satisfactory psychometric properties and, if so, to examine first‐eye, second‐eye, and bilateral cataract surgery outcomes. SETTING: Ophthalmology Clinic, Flinders Medical Centre, Adelaide, Australia. DESIGN: Cohort study. METHODS: The 26‐item VSQ was self‐administered by cataract patients before and after cataract surgery and by those on the cataract surgery waiting list. The VSQ was revised by Rasch analysis using preoperative questionnaire data. The surgical outcome measures were logMAR visual acuities and the Rasch‐refined VSQ. RESULTS: The VSQ was self‐administered by 92 patients before and after surgery (25 first eye, 38 second eye, and 29 bilateral) and by 147 on the waiting list. The preoperative visual acuity (surgical eye) was similar across groups. Cataract surgery improved visual acuities in all groups; the mean better‐seeing eye visual acuity was significantly better (by 2 lines) in second and bilateral surgery patients than in first‐eye surgery patients. Preoperative visual disability was significantly higher in patients awaiting cataract surgery in both eyes than in those awaiting second‐eye surgery. Postoperatively, visual disability decreased significantly in all groups (first eye, 1.66 logits; second eye, 1.52 logits; bilateral, 2.58 logits). However, a floor effect limited the measurement of visual disability after cataract surgery. CONCLUSIONS: The revised visual disability scale of the VSQ measured a reduction in visual disability after first, second, and bilateral cataract surgery. However, the VSQ could not measure lower levels of visual disability, suggesting better questionnaires are required. Financial Disclosure: No author has a financial or proprietary interest in any material or method mentioned.

Divergence in the lived experience of people with macular degeneration.

Purpose The aim of this study was to understand people’s experience with age-related macular degeneration (AMD) in light of new treatment successes. Methods An interpretive qualitative methodology was used to facilitate understanding of the experience of people with AMD. Rich in-depth data were collected using focus groups and individual interviews. Thematic analysis of the data occurred through the processes of line-by-line coding, aggregation, and theme development using the NVivo 10 software. Results A total of 4 focus groups and 16 individual interviews were conducted with 34 people (median age = 81 years; range = 56 to 102 years; 19 females) with AMD. Four major themes arose from the narratives of the participants: cautious optimism, enduring, adaptation, and profound loss. Cautious optimism resonated for participants who had received successful treatment and stabilization of AMD. Enduring emerged as participants with exudative AMD described an ongoing need for invasive and frequent treatments (anti–vascular endothelial growth factor injections) that maintained their vision. Adaptation was evident in the narratives of all participants and was directly related to the physical and psychological limitations that were a consequence of visual disability. Profound loss encompassed both physical and emotional aspects of deteriorating vision and was most evident in patients for whom treatment had failed or had not been considered appropriate for their disease. Conclusions The findings of this study shed new light on the influence of underlying pathology, disease trajectory, and success of new treatments on quality of life of people living with AMD. Optimism toward maintaining vision in the presence of exudative AMD was described by participants, moderated by ongoing caution and a need for endurance of frequent and often problematic intravitreal treatments. These findings add a deeper understanding of this complex and life-changing experience.

Listening to voices of children with a visual impairment: A focus group study

The purpose of the study was to identify the educational, social and leisure activities and issues that matter to school children and young people with a visual impairment and to compare their lifestyle with fully sighted counterparts. Thirteen focus groups were conducted and the groups were stratified by age, gender, visual status and school location (urban and non-urban). The sessions were audio taped, transcribed verbatim, coded using NVivo software and a qualitative data analysis was carried out to identify the main themes. Eighty-one children and young people aged between 5–18 years participated in the focus groups; 34 were visually impaired (22 boys) and 47 were fully sighted (24 boys). In total, 121 different daily living activities important to children and young people were discussed in the focus groups. Results suggested that children and young people with a visual impairment have similar lifestyles to their fully sighted counterparts but are more restricted in some specific activities. The children and young people also reported that sometimes these restrictions were imposed by those supporting them rather than their own abilities. This information provided an in-depth understanding of the impact of visual impairment in school-aged children and young people.