Loss of vision is a form of bereavement, and the loss of vision is mourned.1–4 Adjustment to visual impairment is thought to follow after these initial stages of grieving5 or to exist on a continuum as opposed to existing as a final endpoint.6 Adjustment has been previously described as accommodating to a vision loss by changing one's self-concept and goals to include the realistic restrictions that are imposed because of vision loss, while developing new capabilities that are compatible with personal resources.6,7 However, the concept of adjustment is a complex theory, and a distinct definition of adjustment is lacking. Nevertheless, attempts have been made to define and assess the important factors that are thought to be involved in effective adjustment to visual loss.6,8,9 One such measure of adjustment is the Nottingham Adjustment Scale (NAS),8 which is a self-report instrument that considers the influence of individual psychosocial characteristics in overall effective adjustment to vision loss.
Some of the psychometric properties of the NAS have been evaluated. Item-total correlation analysis of each subscale of the NAS resulted in all subscales exhibiting alpha coefficients between 0.72 and 0.92, indicating a high level of internal consistency.8 Good validity was shown when NAS scores significantly improved as a result of rehabilitative intervention in visually impaired study populations.10–12 The NAS has been used elsewhere in low vision-related12–18 and nonvision-related studies.19,20 However, given recent distinctions,21 the original NAS can be considered as a first generation patient reported outcome (PRO) measure, because its psychometric properties have only been established using classic test theory. The psychometric properties of the NAS have yet to be re-evaluated using Rasch analysis as proposed by guidelines regarding the development of assessment questionnaires in vision science22 to assess response category and item performance, to assess unidimensionality of the construct, and to compute performance measures on an interval scale.
Although it is expected that an individual will experience marked psychosocial burdens at the onset of vision loss, it is apparent that such burdens may not pass, even after receiving low-vision rehabilitation.18,23 In addition, because visual function and visual needs change over time, it is also conceivable that psychosocial function will vary based on individual circumstances and requirements over time. Therefore, it cannot be assumed that subjects with established vision loss are adjusted to their visual circumstances where an endpoint in the adjustment process is met. Because many rehabilitative services such as the provision of low vision aids tend to be implemented indefinitely after the onset of visual loss,24 aspects such as the relative levels of adjustment are still important to consider even in subjects with established vision loss. Assessing the relative levels of adjustment could help to identify any possible barriers to the successful implementation of interventions and used to determine the success of any rehabilitation. Given the recent increased emphasis on emotional support provided to the visually impaired,25 a valid assessment of the relative level of adjustment is required.
Therefore, the aim of this study was to further evaluate whether the NAS could fulfill a role as a satisfactory measure of adjustment to visual impairment. Specifically, Rasch analysis was used to evaluate the instrument's rating scale, item selection, reliability, dimensionality, differential item functioning (DIF), and validity in a sample of people with established vision loss and used to present a valid second-generation PRO measure.
Ninety-nine people participated in the study, which was performed at Essex County and Clacton and District Hospitals and Anglia Ruskin University Eye Clinic. Participants at each location were approached having attended low vision support and rehabilitation sessions and were included if they had experienced vision loss for >6 months, which they felt caused restriction in their daily life.26 The median time since primary ocular diagnosis was 5 years. Those who were younger than age 18 years, were unable to perform verbal evaluations in English, had no perception of light in OU, or were cognitively impaired (as determined by the Mini-Mental State Examination27) were excluded. Ethical approval was granted by Anglia Ruskin University Research Ethics Committee and National Health Service Essex Ethics Committee. The tenets of the Declaration of Helsinki were followed, and all subjects gave informed consent after the nature and possible consequences of the study were explained. All interviews and assessments were performed by the same examiner, a qualified optometrist (DRT).
The 55-Item NAS
Participants completed the original 55-item NAS8,10 as part of a structured face-to-face clinical interview. The original NAS was constructed from previously existing questionnaires relevant to the area of adjustment identified by literature review.8 It consists of eight subscales: anxiety (six questions), depression (six questions), self-esteem (nine statements), attitudes (seven statements), locus of control (four statements), acceptance (nine statements), self- efficacy (eight statements), and attributional style (six statements). Both the anxiety and depression subscales are recorded on a 4-point scale of “not at all,” “no more than usual,” “rather more than usual,” and “much more than usual.” All other subscales consist of statements that are rated on a 5-point response scale of “strongly agree,” “agree,” “do not know,” “disagree,” and “strongly disagree” with a mix of negatively and positively phrased items. In the original instrument, a higher score represents better adjustment. The 55-item NAS is shown in the Appendix at http://links.lww.com/OPX/A31.
The 15-item Geriatric Depression Scale (GDS),28 which uses a dichotomous “yes/no” rating scale, was administered. Binocular distance visual acuity using an externally illuminated Bailey-Lovie chart29 was also assessed with participants wearing any habitual distance correction (chart luminance, 100 cd/m2).
Rasch analysis was undertaken using Winsteps version 22.214.171.124 The Rasch-grouped rating scale model was applied to each rating scale for the analysis (i.e., one Andrich rating scale model applied to anxiety/depression items and another to all other items). Rating scale analysis, and any subsequent scale repair, was undertaken first. Structural calibrations, which indicate the likely level of endorsement of each given response category, were evaluated for each grouping of response scales. Disordered structural calibrations indicate underutilized response categories that can compromise threshold estimate precision.31 Revision of the response scales should be performed to address any disordered structural calibrations.
Fit statistics, which are used to establish the function of individual items in the Rasch model, were reviewed for the entire 55-item NAS. Infit statistics are concerned with responses closer to the mean ability, and outfit statistics are influenced by off-target responses away from the mean ability. Mean-square fit statistics range from zero to infinity where an infit and outfit of 1 represent expected fit of an item to the Rasch model. Values <1 indicate overfit suggesting item redundancy (the item is not adding much extra information to the scale). Values >1 indicate misfit suggesting that items are measuring something other than the proposed Rasch model. Previous recommendations were considered when determining the criteria for item removal.22,32,33 An item was removed from the instrument if it fulfills the following criteria (in order of priority): (1) infit mean square <0.7 or >1.3 and (2) outfit mean square <0.7 or >1.3. The degree of missing data was not considered during item removal, because no item had >10% of missing data.
After any item removal, reliability indices should be evaluated. Person separation statistics provide an indication as to the ability of an instrument to discriminate between respondents, which, in this case, are the ability of the NAS to distinguish participants regarding their level of adjustment. A person separation index of 2.0 (reliability >0.80) has been indicated as a minimum acceptable level.30,34,35 Person-item maps generated by Rasch analysis indicate the targeting of the items in relation to person ability. A difference between mean item and person measures of >1.0 has been considered as notable mistargeting.36
Unidimensionality, i.e., the extent to which an instrument assesses a single latent trait, would then be explored by conducting principal component analysis (PCA) of residuals. Although previous published criteria have cited a minimum level of 60% variance explained by the primary measures as acceptable,36–38 others have not,35 and such a value depends on the variance explained by the person and item measures.30,39,40 An alternative criterion that reflects the construct of an instrument indicating that the estimation of the Rasch measures has been successful would be that the empirical-explained variance of the primary model should approximate that of the model-explained variance.30 Rather than loadings onto a single factor, Rasch-residual-based PCA demonstrates contrasts between opposing factors.30 For there to be considered a second dimension, the first contrast found within the residuals after the primary model has been extracted has to have at least the strength of two items, i.e., an eigenvalue of at least 2.0, because this is close to that seen within random data.30
DIF of the resultant instrument was also assessed. DIF occurs when subjects, across group membership, of the same ability (in this case level of adjustment) respond differently to a given item relative to other items. It is a measure of the stability of the item hierarchy identified by Rasch analysis across groups. DIF was assessed for age (81 years and younger vs. older than 81 years), gender (male vs. female), length of time with ocular condition (≤5 years vs. >5 years), education (standard vs. additional), level of distance binocular visual acuity (≤0.82 logMAR vs. >0.82 logMAR), and living arrangements (alone vs. other). Except for gender and education, groups were determined by performing a median split of the data. Because significance testing for DIF is sample size dependent,41 DIF was classified according to magnitude35–37: small or absent if the difference was <0.50 logits, minimal and probably inconsequential DIF if between 0.50 and 1.0 logits difference, and notable DIF if >1.0 logits difference.
Finally, person measures derived from Rasch analysis of the revised NAS were correlated with person measures generated from Rasch analysis of responses to the 15-item GDS and binocular distance logMAR visual acuity to demonstrate convergent and discriminant validity, respectively.
Table 1 summarizes the descriptive characteristics of the study population.
Rating Scale Analysis
Depression and anxiety questions shared the same rating scale structure (4-point scale: not at all to much more than usual). All other items shared a different rating scale structure (5-point scale: strongly agree to strongly disagree). Analysis of the original rating scale used for anxiety and depression items is shown in Fig. 1a. It can be seen that category 3 (no more than usual) is underused and is never the most probable choice (constituted 14% of responses). Structural calibrations were disordered to reflect this underutilization. Collapsing categories 2 with 3 and 3 with 4 in separate analyses still produced disordered structural calibrations, probably as a result of the additional underutilization of category 1 (much more than usual) and the large ceiling effect exhibited by the end category 4 (not at all). Therefore, categories 1, 2, and 3 were collapsed together to produce a dichotomous scale along with category 4 (Fig. 1b). The categories were relabeled no (category 4) and yes (collapsed categories 1, 2, and 3). Rescaling meant underutilization of categories was minimized and structural calibrations were ordered.
Analysis of the rating scale used for all other subscales of the original NAS is shown in Fig. 2a. It can be seen that category 3 was underused, and structural calibrations were disordered reflecting this underutilization. Category 3 (do not know) is neither closer to agree nor disagree, so logically, it could not be collapsed with either. Therefore, the do not know response of category 3 was deemed to constitute the equivalent of a not applicable response and was instead scored as missing data. This constituted 11% of responses for the rating scale overall and <10% for any individual item within the rating scale (Table 2). The new proposed scoring scale consists of strongly agree, agree, disagree, and strongly disagree. The rescaled categories show ordered structural calibrations (Fig. 2b).
Item Fit and Reduction
After rescaling, Rasch analysis was performed again. Items were then removed one at a time, starting with the most misfitting item (infit mean square, 1.48) and with the analysis reiterated until no items satisfied any removal criteria. This resulted in a 26-item instrument. Despite sufficient person separation (i.e., >2.0), Rasch-based PCA did not support unidimensionality. After extraction of the primary model, two additional contrasts were found in the residuals with eigenvalues of 2.4 and 2.3. Three items (AT1, AT5, and D4) loaded positively (correlation >0.40) on the first contrast, and two items (AC5 and AC7) loaded positively (correlation >0.40) on the second contrast. To establish unidimensionality, these five items were removed.42 After the removal of these items, two further iterations of item removal were required because of item misfit (item LC4 then AS2 removed). For the resulting 19-item instrument, the variance explained in the raw data by the final measures for the empirical calculation (49.7%) was comparable with the variance explained by the model (51.8%). The final instrument was unidimensional in that the first contrast found within the residuals had an eigenvalue of 1.9 (i.e., <2). The retained items are shown in Table 2 and consist of: self-esteem (eight items), attitudes (two items), locus of control (two items), acceptance (four items), and self-efficacy (three items).
Reliability and Targeting of the Resultant Instrument
The person separation reliability coefficient of the revised 19-item instrument, a similar measure to the more traditional Cronbach alpha coefficient,43 is 0.85, and the person separation index is 2.40, indicating good reliability of the person ordering. The item separation reliability coefficient is 0.96. A person-item map is shown in Fig. 3. Compared with the original NAS, the revised instrument shows a change in item targeting against mean person measure, the difference increasing from 0.95 to 1.58 logits. Therefore, the retained items were targeted toward the relatively less adjusted.
Differential Item Functioning
Only two items show notable DIF: LC3 (1.06 logits) is found more difficult by the younger group relative to the other items, and E9 (1.03) is more difficult for those not living alone. Four items demonstrate minimal DIF for age: E9 (0.74) and AC8 (0.57) are found more difficult by younger subjects and SE8 (0.76) and AC3 (0.57) less difficult. Three items show minimal DIF for the length of time with ocular condition: E5 (0.80) and AC3 (0.52) are more difficult for those who have had their condition for a shorter period and SE1 (0.58) less difficult. A single item demonstrates minimal DIF for gender and another for visual acuity: SE8 (0.52) is found more difficult by males, and AC8 (0.53) is more difficult for those with better visual acuity. Four items show minimal DIF for education: AC3 (0.57) and AC8 (0.52) are more difficult for those with additional education, and E1 (0.75) and E9 (0.59) are less difficult relative to the other items.
Convergent and Discriminant Validity
For convergent validity, person measures derived from responses to the revised 19-item instrument are significantly correlated to person measures derived from Rasch analysis of responses to the 15-item GDS (two-tailed Pearson's correlation coefficient, r = −0.56, p < 0.001), where better adjustment is associated with lower levels of depression. With respect to discriminant validity, person measures do not significantly correlate with distance logMAR visual acuity (two-tailed Spearman's correlation coefficient, r = −0.14, p = 0.16).
The results suggest that the performance of the original NAS can be improved by altering the response scale and performing systematic item reduction based on the results of Rasch analysis. The revised 19-item instrument can reliably map adjustment level of the subjects on a unidimensional scale and can also demonstrate stability in the item ordering across different groups. Because the resultant instrument differs significantly from the original NAS in terms of response scale and items, it is retitled as the Acceptance and Self-Worth Adjustment Scale (AS-WAS). The AS-WAS is a second-generation legacy PRO measure,21 assessing self-esteem, attitudes, locus of control, acceptance, and self-efficacy as aspects of adjustment to established visual loss.
Repairing the initial rating scale of the original NAS improved category functions and ordered structural calibrations, helping to avoid category underutilization and improving threshold estimate precision. By using the edited response scales, item reduction improved the overall fit of the remaining items to the Rasch model.
Exploration of the removed items suggests that attributional style does not fit the underlying construct of adjustment here, because all the six items were removed. Attributional style refers to the way an individual ascribes internal or external causes to success and failure (for example item AS1: “Any success I have had has been due to good fortune”). In the scheme of adjustment, attributional style has been questioned previously. Dodds et al.8 demonstrated that attributional style did not significantly correlate with any of the other subscales contained within the original NAS except with self-efficacy and did not significantly improve with rehabilitation.10 As a result, the subscale was omitted from the resultant proposed NAS.10 Furthermore, attributional style as a construct itself has been challenged previously.44
The removal of all depression and anxiety items may also reflect that these constructs are not important in adjustment processes. However, the significant relationship found between GDS scores and adjustment (r = −0.56, p < 0.001) suggests otherwise for depression at the least. Closer inspection of the removed depression and anxiety items reveals high ceiling effects in the end category no (previously “not at all”) potentially leading to their removal from the instrument through item misfit. Such ceiling effects suggest that either low levels of the traits in the current sample or that even the less well adjusted can give a favorable response to these items. We suggest that the latter is more likely, because 23% of the sample showed indications of at least mild depression (raw GDS scores of ≥5), which is near double the rates found in the community dwelling UK elderly population45 and comparable with rates in those who require >6 h of personal care a week because of the conditions such as stroke, heart disease, and severe arthritis.46 The majority of the NAS depression questions can be seen to assess more severe symptoms of depression47 (e.g., item D3: “Have you recently thought of the possibility of doing away with yourself?”) producing higher response ceiling effects and resulting in item misfit. The formulation of items that assess less severe depressive symptoms would be beneficial.
A possible issue with the anxiety items contained within the NAS is that they assess general, nonvision-specific anxiety, having been originally derived from the Golberg General Health Questionnaire.8,48 The items may or may not accurately reflect anxiety symptoms experienced as a result of vision loss. More recently, anxiety measures have been developed for specific vision-related purposes (e.g., Optometric Patient Anxiety Scale),49 where construction of items relating specifically to vision loss with greater face validity appears viable. It is suggested that depression and anxiety are likely to be relevant to adjustment to visual impairment and that to assess adjustment in a broader sense, use of anxiety and depression instruments in conjunction with the AS-WAS should be considered.
The AS-WAS demonstrates a relatively high person separation (G) ratio of 2.40 indicating good precision of measurement for most subjects and suggests that respondents can be reliably discriminated into four groups of adjustment on the basis of their responses [(4G + 1)/3].50 The high person- and item-reliability coefficients (0.87 and 0.94, respectively) imply reliable person and item estimates.
The imperfect targeting of the AS-WAS (1.58 logits) is an issue shared with many other second-generation vision-related self- report instruments.21,35,36,38,42 The item targeting of the AS-WAS suggests that despite acceptable person separation, the scale may further benefit from the formulation of new, more difficult items to discriminate between those who have relatively higher levels of adjustment, potentially increasing the person separation further (Fig. 3).
The final 19-item AS-WAS instrument demonstrated sufficient unidimensionality as the first contrast identified by PCA of the residuals had an eigenvalue of 1.9, which is not greater than would be expected in random data.30 The proposed item ordering of the final model was also found to be stable across different groups in this sample, with no notable DIF (>1.0 logit) present for gender, time since ocular diagnosis, visual acuity, or education. Only two items showed notable DIF (one by age and one by living arrangements), which is likely to be of minimal consequence.
A statistically nonsignificant relationship found between person measures of the revised 19-item AS-WAS instrument and visual acuity is evidence of sufficient discriminant validity. In other words, the instrument measures a realm other than simply the level of visual impairment. Responses to the revised instrument correlate significantly with the level of depressive symptoms as assessed by a well-established instrument, indicating good convergent validity. Because the correlation found (r = −0.56) was not too strong (i.e., r = >0.90), it suggests that the AS-WAS instrument provides additional information to the assessment of depression.
There are two limitations to this study, based on the sample who participated. First, participation in the study was voluntary, and thus selection bias may have existed toward those who are better adjusted. Second, participants were recruited from low-vision clinics with an ethical requirement that their vision loss must have been present for at least the previous 6 months. The median duration since onset of visual impairment was 5 years (range, 0.54 to 64 years). Therefore, the sample predominantly consists of people with established vision loss, as opposed to those in the early stages of visual impairment. This has a number of implications for the results of the study, which are discussed later.
The instrument presented here is only appropriate to be administered to subjects with similar characteristics to the current sample, i.e., those with established visual impairment. However, considering that, for example, at least 50% of all appointments to the low-vision clinic at Moorfields Eye Hospital between 1973 and 2003 were for existing visually impaired patients,24 it is likely that subjects with established vision loss represent a large proportion of patients seen at many low-vision clinics.
It is unclear whether adjustment to visual impairment occurs as a continual process6 or as one which results in an endpoint being reached.5 With reference to the World Health Organization's definition of disability,51 the diagnosis of an ocular disease or pathology is not the cause of a disability; rather, it is the functional limitations as a result of the visual impairment caused by the condition. Therefore, the length of time with the resultant functional limitations may be more important in adjustment than the actual time with the ocular condition. Such a measure of functional limitations is difficult to assess as needs and requirements in daily life vary on an individual basis often regardless of the severity of visual loss52 and are likely to alter over time with changing visual function (e.g., implementation of compensatory strategies vs. deterioration of ocular morbidity). Similar to functional needs, adjustment might, therefore, be seen as a continuous process without a definite endpoint,6,53 the relative level of which varies depending on individual needs and circumstances, regardless of visual acuity and length of time since diagnosis.
There is some evidence to support the premise that adjustment occurs as a continual process, which needs frequent reevaluation. In a previous study, >65% of patients with low vision reported emotional difficulties despite having established visual impairment (mean, 8.6 years), and improved ways of accepting visual loss and building confidence were identified as additional rehabilitative requirements.54 In addition, there has been little evidence of psychosocial adjustment in patients with age-related macular degeneration, despite at least a year elapsing from the time of referral to low-vision clinics.18,23 In this study, all participants were attending for visual support and rehabilitation despite having relatively established vision loss, suggesting at the least that their functional needs and circumstances were not constant. Furthermore, item ordering of the final instrument was stable regardless of severity and length of time since ocular diagnosis (i.e., no notable DIF), and the severity and length of time were not significantly associated with adjustment (two-tailed Spearman's coefficient, r = −0.14, p = 0.16 and r = 0.06, p = 0.55, respectively).
The current assessment of adjustment in people with established vision loss implies that the process of adjustment may be a continuum, rather than an endpoint that can be reached. Nevertheless, further research is required before conclusions can be made regarding the pathway of adjustment. From this study, all that can be concluded is that aspects of adjustment concerned with acceptance, attitudes, and self-worth (i.e., self-efficacy, self-esteem, and locus of control) can be assessed on a unidimensional scale using the presented 19-item AS-WAS with as much as a 5.5-logit range of adjustment scores even in those with predominantly established vision loss.
Because the pathway of adjustment is still largely unclear, the results of this study do not allow for generalizing to those with newly acquired vision loss. Assessing the original NAS on a sample of subjects with newly acquired vision loss would confirm whether the item hierarchy, response scale repairs, and item removal iterations are similar for these subjects as the results presented here. However, the purpose of this study has been to validate an instrument for use in a population with established visual impairment.
The final 19-item AS-WAS is indicated for use as a second-generation PRO measure21 to be implemented in subjects with established visual loss to assess the level of adjustment concerned with acceptance, attitudes, and self-worth. The AS-WAS, rather than assuming that adjustment has occurred as an endpoint, can assess these relevant parameters to determine if such aspects of adjustment are a barrier to the rehabilitation of a patient. Similarly to other PROs, the revised instrument can also be used to determine the effects of any interventions such as different approaches to low-vision rehabilitation (e.g., emotional vs. instrumental support), to determine the effects of any other visual therapy, and to study an individual's adjustment profile over time to help improve understanding of the process. An additional depression and anxiety scale is recommended for use in conjunction with the AS-WAS if assessment of adjustment in a broader sense is required.
In conclusion, within this study sample, the original NAS is not psychometrically optimal when assessing the level of adjustment. Its functioning can be improved by altering the response scale and by systematic item removal based on Rasch analysis. The revised 19-item AS-WAS is unidimensional, measuring the aspects of adjustment concerned with acceptance, attitudes, and self-worth. The AS-WAS has good reliability, convergent, and discriminant validity and is stable across groups. The findings of this study indicate that the level of adjustment is important to consider even in established vision loss and that future research should consider differences in adjustment between those with new and with established visual impairment.
This work was supported by a College of Optometrists' Scholarship (to DRT).
Department of Vision and Hearing Sciences
Anglia Ruskin University
Cambridge CB1 1PT, United Kingdom
An appendix is available online at http://links.lww.com/OPX/A31. Cited Here...
1.Berman K, Brodaty H. Psychosocial effects of age-related macular degeneration. Int Psychogeriatr 2006;18:415–28.
2.Blank HR. Psychoanalysis and blindness. Psychoanal Q 1957;26:1–24.
3.Cholden L. Some psychiatric problems in the rehabilitation of the blind. Bull Menninger Clin 1954;18:107–12.
4.De Leo D, Hickey PA, Meneghel G, Cantor CH. Blindness, fear of sight loss, and suicide. Psychosomatics 1999;40:339–44.
5.Dodds AG. The concept of adjustment: a structural model. J Vis Impair Blind 1994;88:487–97.
6.Horowitz A, Reinhardt JP. Development of the adaptation to age-related vision loss scale. J Vis Impair Blind 1998;92:30–41.
7.Moore JE. Impact of family attitudes toward blindness/vision impairment on the rehabilitation process. J Vis Impair Blind 1984;78:100–6.
8.Dodds AG, Bailey P, Pearson A, Yates L. Psychological factors in acquired visual impairment: the development of a scale of adjustment. J Vis Impair Blind 1991;85:306–10.
9.Reinhardt JP. Effects of positive and negative support received and provided on adaptation to chronic visual impairment. Appl Dev Sci 2001;5:76–85.
10.Dodds AG, Flannigan H, Ng L. The Nottingham Adjustment Scale: a validation study. Int J Rehabil Res 1993;16:177–84.
11.Dodds AG, Ng L, Yates L. Residential rehabilitation: 2. Psychological outcome of rehabilitation. New Beacon 1992;76:273–377.
12.Pankow L, Luchins D, Studebaker J, Chettleburgh D. Evaluation of a vision rehabilitation program for older adults with visual impairment. Top Geriatr Rehabil 2004;20:223–32.
13.Kef S. Psychosocial adjustment and the meaning of social support for visually impaired adolescents. J Vis Impair Blind 2002;96:22–37.
14.Nelson P, Aspinall P, Papasouliotis O, Worton B, O'Brien C. Quality of life in glaucoma and its relationship with visual function. J Glaucoma 2003;12:139–50.
15.Rimmerman A, Morgenstern H. Quality of life of visually impaired adults who are employed in extended-employment programs in Israel. J Vis Impair Blind 2003;97:551–61.
16.Frost NA, Sparrow JM, Durant JS, Donovan JL, Peters TJ, Brookes ST. Development of a questionnaire for measurement of vision-related quality of life. Ophthalmic Epidemiol 1998;5:185–210.
17.Wolffsohn JS, Eperjesi F. Predicting prescribed magnification. Ophthalmic Physiol Opt 2004;24:334–8.
18.Reeves BC, Harper RA, Russell WB. Enhanced low vision rehabilitation for people with age related macular degeneration: a randomised controlled trial. Br J Ophthalmol 2004;88:1443–9.
19.Koyama Y, Miyashita M, Kazuma K, Suzukamo Y, Yamamoto M, Karita T, Takatori Y. Preparing a version of the Nottingham Adjustment Scale (for psychological adjustment) tailored to osteoarthritis of the hip. J Orthop Sci 2006;11:359–64.
20.Suzukamo Y, Ohbu S, Kondo T, Kohmoto J, Fukuhara S. Psychological adjustment has a greater effect on health-related quality of life than on severity of disease in Parkinson's disease. Mov Disord 2006;21:761–6.
21.Pesudovs K. Item banking: a generational change in patient-reported outcome measurement. Optom Vis Sci 2010;87:285–93.
22.Pesudovs K, Burr JM, Harley C, Elliott DB. The development, assessment, and selection of questionnaires. Optom Vis Sci 2007;84:663–74.
23.Stanford P, Waterman HA, Russell WB, Harper RA. Psychosocial adjustment and quality of life in age related macular degeneration. Br J Visual Impair 2009;27:129–46.
24.Crossland MD, Silver JH. Thirty years in an urban low vision clinic: changes in prescribing habits of low vision practitioners. Optom Vis Sci 2005;82:617–22.
27.Folstein MF, Folstein SE, McHugh PR. “Mini-mental state.” A practical method for grading the cognitive state of patients for the clinician. J Psychiatr Res 1975;12:189–98.
28.Yesavage JA, Brink TL, Rose TL, Lum O, Huang V, Adey M, Leirer VO. Development and validation of a geriatric depression screening scale: a preliminary report. J Psychiatr Res 1982;17:37–49.
29.Bailey IL, Lovie JE. New design principles for visual acuity letter charts. Am J Optom Physiol Opt 1976;53:740–5.
30.Linacre JM. WINSTEPS Rasch Measurement [computer program], Version 3.69.1. Chicago, IL: Winsteps.com
31.Linacre JM. Investigating rating scale category utility. J Outcome Meas 1999;3:103–22.
32.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.
33.Wright BD, Linacre JM. Reasonable mean-square fit values. Rasch Measurement Transactions 1994;8:370. Available at: http://www.rasch.org/rmt/rmt83b.htm
. Accessed November 11, 2009.
34.Bond TG, Fox CM. Applying the Rasch Model: Fundamental Measurement in the Human Sciences, 2nd ed. Mahwah, NJ: Lawrence Erlbaum Associates Publishers; 2007.
35.Gothwal VK, Wright TA, Lamoureux EL, Pesudovs K. Rasch analysis of the quality of life and vision function questionnaire. Optom Vis Sci 2009;86:836–44.
36.Gothwal VK, Wright TA, Lamoureux EL, Pesudovs K. Rasch analysis of visual function and quality of life questionnaires. Optom Vis Sci 2009;86:1160–8.
37.Marella M, Gothwal VK, Pesudovs K, Lamoureux E. Validation of the visual disability questionnaire (VDQ) in India. Optom Vis Sci 2009;86:826–35.
38.Gothwal VK, Wright T, Lamoureux EL, Pesudovs K. Validity of the adaptation to age-related vision loss scale in an Australian cataract population. J Optom 2009;2:142–7.
41.Hambleton RK. Good practices for identifying differential item functioning. Med Care 2006;44:S182–8.
42.Gothwal VK, Wright T, Lamoureux EL, Pesudovs K. Activities of Daily Vision Scale: what do the subscales measure? Invest Ophthalmol Vis Sci 2010;51:694–700.
43.Cronbach LJ. Coefficient alpha and the internal structure of tests. Psychometrika 1951;16:297–334.
44.Cutrona CE, Russell D, Jones RD. Cross-situational consistency in causal attributions: does attributional style exist? J Pers Soc Psych 1984;47:1043–58.
45.Osborn DP, Fletcher AE, Smeeth L, Stirling S, Nunes M, Breeze E, Siu-Woon Ng E, Bulpitt CJ, Jones D, Tulloch A. Geriatric Depression Scale Scores in a representative sample of 14 545 people aged 75 and over in the United Kingdom: results from the MRC Trial of Assessment and Management of Older People in the Community. Int J Geriatr Psychiatry 2002;17:375–82.
46.Chiang KS, Green KE, Cox EO. Rasch analysis of the geriatric depression scale-short form. Gerontologist 2009;49:262–75.
48.Goldberg D. The General Health Questionnaire. Windsor, UK: NFER-Nelson Publishing Company; 1981.
49.Court H, Greenland K, Margrain TH. Content development of the optometric patient anxiety scale. Optom Vis Sci 2007;84:729–37.
50.Wright BD, Masters GN. Rating Scale Analysis: Rasch Measurement. Chicago, IL: MESA Press; 1982.
52.Haymes SA, Johnston AW, Heyes AD. Relationship between vision impairment and ability to perform activities of daily living. Ophthalmic Physiol Opt 2002;22:79–91.
53.Wahl HW, Becker S, Burmedi D, Schilling O. The role of primary and secondary control in adaptation to age-related vision loss: a study of older adults with macular degeneration. Psychol Aging 2004;19:235–9.
54.Rees G, Saw CL, Lamoureux EL, Keeffe JE. Self-management programs for adults with low vision: needs and challenges. Patient Educ Couns 2007;69:39–46.