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.
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adjustment; Rasch analysis; visual impairment; questionnaire; quality of life; low vision
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