Tài liệu Improving equitable access to cataract surgery in rural southern china using willingness to pay data to assess the feasibility of a tiered pricing model to subsidize surgeries to the poorest

IMPROVING EQUITABLE ACCESS TO CATARACT SURGERY IN RURAL SOUTHERN CHINA: USING WILLINGNESS TO PAY DATA TO ASSESS THE FEASIBILITY OF A TIERED PRICING MODEL TO SUBSIDIZE SURGERIES TO THE POOREST by Elaine M Baruwa A dissertation submitted to Johns Hopkins University in conformity with the requirements for the degree of Doctor of Philosophy Baltimore, Maryland June 2007 © Elaine M Baruwa 2007 All Rights Reserved UMI Number: 3288601 Copyright 2007 by Baruwa, Elaine M. All rights reserved. INFORMATION TO USERS The quality of this reproduction is dependent upon the quality of the copy submitted. Broken or indistinct print, colored or poor quality illustrations and photographs, print bleed-through, substandard margins, and improper alignment can adversely affect reproduction. In the unlikely event that the author did not send a complete manuscript and there are missing pages, these will be noted. Also, if unauthorized copyright material had to be removed, a note will indicate the deletion. ® UMI UMI Microform 3288601 Copyright 2008 by ProQuest Information and Learning Company. All rights reserved. This microform edition is protected against unauthorized copying under Title 17, United States Code. ProQuest Information and Learning Company 300 North Zeeb Road P.O. Box 1346 Ann Arbor, Ml 48106-1346 Abstract Title: Improving Equitable Access to Cataract Surgery in Rural Southern China: Using Willingness to Pay Data to Assess the Feasibility of a Tiered Pricing Model to Subsidize Surgeries to the Poorest. Aim: To assess the equity of financial access to cataract surgery given willingness to pay (WTP) for cataract surgery at the current price of surgery and for added amenities such as surgery by a senior surgeon, an improved intraocular lens, transport and food. To determine the feasibility of a tiered pricing and cross-subsidization model using these estimates. Methods: A WTP survey was administered at community screenings and hopsital cataract surgery clinics in rural Guangzhou. WTP was estimated using interval regression and then compared to the price of surgery to determine access. A further equity analysis was conducted using concentration indices and curves. The WTP for amenities was similarly analyzed to determine potential demand. Results: WTP surveys were conducted with 656 patients and 342 of their caregivers. The mean WTP for the community screening patients was 371RMB (S.D. 114RMB) and 570RMB (S.D. 69RMB) for the hospital patients (8RMB =US$1). For caregivers the mean was 619 RMB (S.D. 77 RMB). At the two prices charged by HKI, 500RMB and 630RMB, the estimated concentration indices were 0.18 and 0.36 for patients, which implies that financial access is inequitably concentrated amongst the wealthier patients. However, the respective index measures were 0.01 and 0.10, for caregivers indicating lower inequity at 630RMB and no inequity at 500RMB. The WTP for amenities was low, only 78RMB for a n senior surgeon and 42RMB for an improved IOL. Conclusion: Access to cataract surgery is inequitably distributed between the poor and the poorest in this population even at cost, 500RMB. We determined that not enough patients would be able to purchase surgery at higher, tiered prices for additional amenities in order to subsidize any significant number of surgeries at a lower price. While WTP for cataract surgery was significantly higher when assessed by patient's caregivers, adjusting for this did not change the finding that access is inequitable for this population and creative ways must be found to lower prices. Thesis Committee: Kevin Frick, PhD, Department of Health Policy and Management, JHSPH David Bishai, MD PhD, Department of Population and Family Health, JHSPH Emily West Gower, PhD, Department of Ophthalmology, JHMI Damian Walker, PhD, Department of International Health, JHSPH Laura Morlock, PhD, Department of Health Policy and Management, JHSPH in ACKNOWLEDGEMENTS I would like to thank: The Department of International Health, JHSPH for the excellent teaching and support that they gave me during my doctoral studies. In particular, Carol Buckley for her all her help, making sure that I never got lost administratively. My colleagues at the PneumoADIP for their encouragement and my director Angeline Nanni, for understanding my priorities and accomodating them with such empathy. My fellow doctoral students were an invaluable source of encouragement and friendship, particularly during both of my pregnancies. Arantxa Colchera, Nhan Tran, Marjorie Opuni, Rebekah Heinzen and Tram Lam studied with me, baby-sat for me, pondered the pros/cons of doctoral studies (mostly the cons), and attended my defense. My family: Chiadi, Ketandu and Omenka for being so patient with a wife and mother who seemed to always have too much to do at the same time. I love them so much. Their smiles and laughter kept me going on the rare occasions when I did feel as though 24 hours in a day and a single brain were not quite enough to get throught this journey. Finally my advisor, Kevin Frick, who is extremely bright, seems to have 36 hours in work day and possesses a bizarre affection for econometrics but his patience, his work ethic and his generosity have been inspirational to me. I aspire to be the type of mentor, teacher and friend that he has been to me and consider myself truly blessed to shared this experience with him. TABLE OF CONTENTS 1 2 3 4 5 STUDY AIM AND OBJECTIVES 1 1.1 OBJECTIVE 1 3 1.2 OBJECTIVE 2 3 1.3 OBJECTIVE 3 4 BACKGROUND 5 2.1 EPIDEMIOLOGY OF CATARACT AND CATARACT SURGERY 5 2.2 RURAL HEALTH CARE IN CHINA 6 2.3 HKI, CHINA AND TIERED PRICING 10 2.4 THE ARAVIND EYE HOSPITAL, INDIA 11 2.5 HKI, CHINA AND CATARACT SURGERY 13 CONCEPTUAL FRAMEWORK 14 3.1 DEFINING EQUITY IN TERMS OF WILLINGNESS TO PAY 14 3.2 SOCIAL WELFARE AND THE EQUITY-EFFICIENCY TRADE-OFF 16 3.3 THE ECONOMICS OF TIERED PRICING 18 CONTINGENT VALUATION AND WILLINGNESS TO PAY 20 4.1 CONTINGENT VALUATION 20 4.2 WTP AND 'DEMAND' 23 4.3 WTP AND SOCIAL WELFARE 25 4.4 WTP AND EXTERNALITIES 27 4.5 WTP AND ALTRUISM 29 THE USE OF WTP IN DEVELOPING COUNTRY RESEARCH 31 5.1 WTP FOR INSECTICIDE TREATED BEDNETS IN EASTERN NIGERIA 31 5.2 WTP FOR COMMUNITY-BASED INSURANCE IN BURKINA FASO 32 v 6 7 8 9 5.3 WTP FOR CATARACT SURGERY IN NEPAL 33 5.4 WTP FOR CATARACT SURGERY IN TANZANIA 33 5.5 FINDINGS AND IMPLICATIONS 34 5.6 BEST PRACTICE FOR WTP SURVEY ADMINISTRATION 37 DATA COLLECTION 42 6.1 SAMPLING FRAMEWORK 42 6.2 SAMPLE SIZE 43 6.3 SURVEY DESIGN 44 6.4 SURVEY ADMINISTRATION 50 STATISTICAL METHODS 53 7.1 CATEGORICAL OUTCOMES - INTERVAL REGRESSION 53 7.2 CONCENTRATION CURVE AND INDEX ESTIMATION 58 RESULTS 63 8.1 SAMPLE SIZE AND RESPONSE RATE 63 8.2 SAMPLE CHARACTERISTICS 65 8.3 BIVARIATE ASSOCIATIONS WITH WTP ANYTHING FOR CATARACT SURGERY 75 8.4 MAXIMUM WILLINGNESS TO PAY FOR CATARACT SURGERY 79 8.5 OBJECTIVE 1 PATIENTS WILLINGNESS TO PAY 86 8.6 OBJECTIVE 2 CAREGIVERS WILLINGNESS TO PAY 99 8.7 EQUITY OF ACCESS USING CAREGIVER'S WTP 104 8.8 OBJECTIVE 3 WILLINGNESS TO PAY FOR AMENITIES 105 DISCUSSION 109 9.1 FACTORS AFFECTING PATIENT'S WTP 109 9.2 PATIENT'S WILLINGNESS TO PAY 113 9.3 FACTORS AFFECTING CAREGIVER'S WTP 115 vi 9.4 HOUSEHOLD CHARACTERISTICS' IMPACT ON WTP ON PAIRED RESPONDENTS 116 9.5 CAREGIVERS' PREDICTED WILLINGNESS TO PAY 118 9.6 EQUITY OF ACCESS 122 9.7 POLICY IMPLICATIONS FOR HKI 124 9.8 POLICY IMPLICATIONS FOR CHINA'S 9.9 WAS THE METHODOLOGY APPROPRIATE FOR OUR OBJECTIVES? 128 9.10 WAS THE METHODOLOGY APPROPRIATE FOR THIS POPULATION? 128 9.11 BEST PRACTICE FN PRACTICE 133 9.12 STUDY LIMITATIONS 137 9.13 CONCLUSION 140 10 APPENDICES RCMS 127 151 10.1 WTP SURVEY FOR PATIENTS 151 10.2 WTP SURVEY FOR CAREGIVERS 171 11 CURRICULUM VITAE- ELAINE MONISOLA BARUWA 190 VII TABLE OF TABLES TABLE 1 NEW COMMUNITY MEDICAL SCHEME - PREMIUMS, CO-PAYMENTS AND DEDUCTIBLES 9 TABLE 2 SURVEY STRUCTURE 45 TABLE 3 SAMPLE SIZE BY SITE AND TYPE 63 TABLE 4 SAMPLE SOCIODEMOGRAPHICS 66 TABLE 5 WORK STATUS AND CARE REQUIREMENTS 67 TABLE 6 VISUAL ACUITY CLASSIFICATION 69 TABLE 7 SAMPLE VISUAL ACUITY 69 TABLE 8 SAMPLE HOUSEHOLD INCOME 71 TABLE 9 REASONS FOR NOT WANTING TO PAY FOR SURGERY 72 TABLE 10 FIRST PAYMENT CARD AS A DETERMINANT OF MAXIMUM WTP 74 TABLE 11 BIVARIATE ASSOCIATIONS WITH WILLINGNESS TO PAY ANYTHING FOR CATARACT SURGERY (PATIENTS ONLY, TABLE 12 MAXIMUM N=656) WTP ANYTHING FOR CATARACT SURGERY, 76 (N=656) 79 TABLE 13 NUMBER OF RESPONDENTS AND THEIR MAXIMUM EXPRESSED WTP BY PAYMENT CARD AND BY SITE 84 TABLE 14 CHECKING THE CONSISTENCY OF IMPUTED INCOME VARIABLES 86 TABLE 15 MAXIMUM WTP - FINAL PATIENT MULTIVARIATE MODEL (N=656) 89 TABLE 16 PREDICTED WTP AND ACCESS FOR PATIENTS 92 TABLE 17 CONCENTRATION INDICES FOR PATIENTS 96 TABLE 18 CAREGIVER MAXIMUM WTP MODEL 101 TABLE 19 PREDICTED MAXIMUM WTP FOR PAIRS 103 TABLE 20 CONCENTRATION INDEX FOR CAREGIVERS 105 TABLE 21 WILLINGNESS TO PAY FOR AMENITIES 106 TABLE 22 MAXIMUM WTP FOR A SENIOR SURGEON FROM PATIENTS 107 TABLE 23 PREDICTED WILLINGNESS TO PAY FOR AMENITIES 107 viii TABLE 24 HOUSEHOLD SIZE AND NUMBER OF CHILDREN TABLE 27 CONCENTRATION CURVE FOR 250RMB SURGERY FOR COM. SCREENING PATIENTS 117 125 IX TABLE OF FIGURES FIGURE 1 CONCENTRATION CURVE - EQUITABLE ACCESS 59 FIGURE 2 FIRST PAYMENT CARD ASKED BY INTERVIEWER 74 FIGURE 3 UNADJUSTED MAXIMUM WTP BY SITE 85 FIGURE 4 PREDICTED WTP BY SITE 94 FIGURE 5 ACCESS TO CATARACT SURGERY AT 500RMB SHOWING THE % IN NUMBERS OF RESPONDENTS BY SITE FIGURE 95 6 ACCESS TO CATARACT SURGERY BY INCOME AT 630RMB SHOWING THE % IN NUMBERS OF RESPONDENTS BY SITE CURVES FOR PATIENTS AT 500RMB BY SITE 98 FIGURE 8 CONCENTRATION CURVES FOR PATIENTS A T 6 3 0 R M B BY SITE 99 FIGURE 7 CONCENTRATION 96 FIGURE 9 COMPARISON OF UNADJUSTED MAXIMUM WTP RESPONSES BY PAIRS 100 FIGURE 10 ACCESS TO CATARACT SURGERY BY CAREGIVERS 104 FIGURE 11 CONCENTRATION CURVE FOR PATIENTS AND CAREGIVERS AT 500RMB 105 FIGURE 12 SAMPLE GENDER (A) AND ACCESS BY GENDER (B) 111 FIGURE 13 SAMPLE EDUCATION AND ACCESS BY EDUCATION 112 FIGURE 14 WTP FOR CATARACT SURGERY FOR MATCHED PAIRS (CHILD IS PAYMENT SOURCE) .... 120 FIGURE 16 WTP RESPONSES BY SITE 131 FIGURE 17 PREDICTED MAXIMUM WTP AS A % OF ANNUAL HH INCOME 132 x 1 Study Aim and Objectives This study was designed to evaluate whether access to cataract surgery is equitable in the Guangdong Province of the People's Republic of China (PRC) and to explore the feasibility of using a tiered pricing model to increase uptake by the poorest, using data from a willingness to pay survey administered to a rural population in this region. China and Cataract Cataract is the leading cause of blindness in the PRC in people aged 50 and over. Prevalence rates of cataract blindness have been estimated to be up to 4.37%, with rates of low vision being even higher in this age group (Hsu, Cheng, Liu, Tsai, & Chou, 2004; Li, Xu, He, Wu, Munoz, & Ellwein, 1999a). Combined with a low cataract surgery rate of 230 per million per year the result is that China has a severe burden of curable blindness and low vision (Apple, Ram, Foster, & Peng, 2000). Helen Keller International, China Helen Keller International (HK.I), in conjunction with the privately owned Guangming Eye Hospital (GEH) and the Yang Jiang local government health department set up a cataract screening and surgery program in 2001. The program now provides about 1800 surgeries a year which translates roughly to a rate of at least 720 per million if we do not include the surgeries performed by other providers. A cross sectional willingness to pay study conducted three months after the program began, suggested that income would be a limiting factor for access to cataract surgery even with the service priced at cost (He M et al., 2007). Now the program would like to determine whether or not it is feasible to use a tiered pricing structure to increase its revenues in order for it to provide cataract surgery at a lower price to those 1 unable to pay the current fee of 500 - 630 Renminbi (RMB) where 1 US$=8RMB. Access: Inequality and Inequity In the 2001 study it was found that there were significant differences in the amount that respondents were willing to pay across income groups, specifically, those in higher income groups were willing to pay higher amounts. This finding highlights an inequality in willingness to pay that is not necessarily inequitable - there is nothing 'unfair' about individuals with a higher income being willing to spend more than individuals with lower income. However it was also found that only 37% of the respondents would be willing to pay 500RMB or more to obtain cataract surgery. This result suggests that even though the service is now available to this population, there may remain access limitations for some individuals due to the pricing and this outcome is inequitable. The combination of these findings suggests that, with enough income variation, it might be possible to induce those with higher incomes who may be willing to pay more for surgery, to actually do so and then to use the increased revenue to subsidize a lower price that improves access for those with lower incomes and willingness to pay. In other words we could take advantage of an existing income inequality and provide somewhat unequal services to reduce an access inequity for the most basic level of service. This study will utilize data from a willingness to pay survey to obtain a valuation of cataract surgery by respondents and their caregivers which, when combined with the known prices, will determine whether or not access is equitable. It will then determine whether or not a large enough number of respondents value additional amenities highly enough to enable higher pricing. Such amenities could include having a senior surgeon perform their surgery, having an improved intra-ocular lens implanted or having food and transport provided for 2 them. With estimates of revenue, a range of possible subsidized prices can be determined and used to predict the impact of the model on the equity of access to cataract surgery. 1.1 Objective 1 To determine whether access to cataract surgery is equitable in this population using willingness to pay survey data from respondents with cataract Empirical Analysis: The results from a survey administered to respondents who are cataract blind in at least one eye will be used to explore how willingness to pay for cataract surgery may differ by demographic and socioeconomic characteristics, vision status and potential sources of payment. Following this exploration, an appropriate model to estimate willingness to pay will be proposed and tested. From these results an 'incidence rate' for cataract surgery at current pricing levels will be determined and combined with the income data to construct a concentration index that describes the equity of access. 1.2 Objective 2 To determine whether willingness to pay differs between respondents with cataract and their households/caregivers and what impact this has upon the willingness to pay estimates Empirical Analysis: The results from a survey administered to the caregivers who accompany respondents will be used to explore how willingness to pay for cataract surgery may differ between patients and another member of their household and to determine how much care the patient needs because of their impaired vision. Specifically this comparison will be used: l) To determine why there may be differences in WTP from respondents who come from 3 the same household and are subject to the same income constraint. It could be important if sources other than own savings and insurance are used to pay for surgery 2) To determine if there are intergenerational differences in the perceived need for surgery 3) To determine whether there are differences in perceived control of household resources 4) To determine whether a societal valuation of cataract surgery might be significantly higher or lower than the patients' valuation of cataract surgery 1.3 Objective 3 To estimate the revenue that can be expected from a tiered pricing model and to determine the potential of the model to improve equity of access to surgery Empirical Analysis: The willingness to pay data will be used to assess the potential demand for the additional amenities that GEH/HKI could provide at minimal cost. These amenities are having a senior surgeon perform the surgery, an improved intraocular lens, transport to/from the clinic and the provision of meals. Subsequently, the projected revenue from the provision of such services will be estimated and used to determine the feasibility of a tiered pricing and cross subsidization model. To avoid having to determine patients' choices between amenities, revenues will be determined from the provision of a single amenity at a time. Finally a concentration index will be estimated at each feasible subsidized price to see what impact this model may have on the equity of access. 4 2 Background 2.1 Epidemiology of Cataract and Cataract Surgery Cataract is the opacification of the lens and its major risk factor is aging. Other risk factors postulated include diabetes, smoking, alcohol and UVB exposure (Cataract: Epidemiology and service delivery.2000). Cataract can occur unilaterally but is more often bilateral, with the cataract in each eye likely to develop and worsen vision at differing rates. Surgical removal of the lens is the only treatment. There are different methods for cataract extraction and costs are very dependent upon which method is used and whether the lens is replaced or the patient is given aphakic spectacles to improve their post surgery vision. Global estimates of visual impairment by the WHO are that I6l million people were blind or had low vision in 2002 and 90% of these people live in developing countries. Despite being treatable, cataract is the leading cause of visual impairment by far and causes 48% of global blindness compared to the next largest cause glaucoma which causes 12%( World Health Organization, November 2004). Blindness is defined as having visual acuity of less than 0.05 in the better eye with best possible correction. That is, being able to see at 3 meters what a person with normal vision can see at 60 meters or 3/60 in Snellen Visual Acuity in meters. Low vision is defined as visual acuity of less than 0.33 (20/60), but equal to or better than 3/60'. In terms of ' By the 10th Revision of the WHO International Statistical Classification of Disease, Injuries and Causes of Death. 5 functional vision , consider that in the state of Maryland a driver can obtain an unrestricted license with a minimum visual acuity of 10/20 or 0.5 in either eye and the legal definition of blindness in the United States is 20/200. Cataract is the leading cause of blindness in people over the age of 50 in China where there are approximately 90 million people over the age of 60 (Li, Xu, He, Wu, Munoz, & Ellwein, 1999a; Zhang et al., 1992; Zhang, Zou, Gao, Di, & Wang, 1992). The estimated prevalence of blindness in China is estimated to be between 2.94% and 4.37% and estimates of low vision are even higher (Hsu et al., 2004; Li, Xu, He, Wu, Munoz, & Ellwein, 1999a). The cataract surgery rate (CSR) varies across China and was estimated to be 138 per million per year in Guangdong circa 1992. It was as low as 28 in Hebei and as high as 1500 in Xizang. The current average of 230-320 across China is very low when compared to India where the CSR is 3650 despite having a cataract surgeon to total population ratio that is comparable to China's (Foster, 2001; Li, Xu, He, Wu, Munoz, & Ellwein, 1999a; Zhao, Sui, Jia, Fletcher, & Ellwein, 1998). 2.2 Rural Health Care in China Yang Jiang is a county of the Guangdong Province comprised of 1 urban city and 3 rural sub-counties, located on the South East coast of China. The Guangdong Province is one of the wealthiest in China by virtue of its economically beneficial coastline, which has given rise Functional vision will be described in more detail in the Results section in order to illustrate the level of visual impairment in the population sample. 6 to massive swathes of industrialization. However, there is severe inequality between the urban and rural economies. Income estimates for all the Central Provinces, including Guangdong are 7,900 RMB annual per capita in urban areas and 2,652 RMB annual per capita in rural areas (Sicular T., Yue X., Gustafsson B., & Li S., 2006). Rural Guangdong is as poor as rural regions across China. The Old Rural Cooperative Medical System Until the late 1970s about 90% of China's rural villages were well served by the Rural Cooperative Medical System (RCMS) that was a pre-payment plan financed by household premiums, village level collective welfare funds and a small amount of higher level government funding. This system, famous for its 'barefoot doctors,' is widely acknowledged as being a tremendous success. It is credited with making a major contribution to China's first 'health care revolution' in which life expectancy increased from 38 in 1949 to 68 in 1978. With the move from central planning toward a market economy, the commune system moved to a 'household responsibility' system. With the removal of the risk-sharing benefit of the collective welfare fund, the majority of RCMS funds collapsed and by 1998 only 9.5% of the rural population was insured. The government maintained its level of very limited investment in rural health care services, which combined with a high level of financial decentralization, left the rural population with severely limited access to care. Local county, township and village health posts are available, but they have little impact on access to healthcare for the poor because they receive such limited government subsidization. Yang Jiang General Hospital for example, has a turnover of 170 million RMB but receives only 300,000 RMB in 7 subsidy from the government3. In order to ensure access to basic care and equity, government 'mandates' prices with little regard to the feasibility of maintaining services. Consequently there is little incentive to provide basic services. The need to remain financially stable encourages the practice of lucrative drug over-prescription and unnecessary/avoidable expensive procedures. At the village level there is no government subsidy at all and so the providers of basic services are essentially private providers of care, working on a fee for service basis. The New RCMS The scheme that replaced the RCMS proved to be largely unsustainable due to the lack of central government financial participation. It was modified into the New Community Medical System (NCMS) that now incorporates a matched financing model to encourage enrollment and increase sustainability. Specifically, enrollees pay 10 RMB per capita per year, which is matched by 20 RMB by the central government and at least 20 RMB by the local government (40 RMB in the wealthier provinces). The matching is entirely reliant on the enrollee's contribution. Uptake has been estimated at 70% in the pilot schemes that cover about 65 million of China's 450 million rural dwellers (Wagstaff A, Lindelow M, Jun G, Ling X, & Juncheng Q, 2007). The program is set to begin expansion to cover 80% of the country's rural population in 2007. When paid, the premium entitles the rural enrollee to discounts at health service institutions, both private and public, in the form of co-pays for services, with a maximum of 3000 RMB per year beyond which fees are out of pocket. Personal communication, Dr He, Director, Guangming Eye Hospital 8 Table 1 shows how the NCMS operates in rural Yang Jiang and how the urban health insurance works for those seeking cataract surgery (there are no 'rural' reimbursable providers of cataract surgery). Table 1 New Community Medical Scheme - Premiums, Co-payments and Deductibles Annual Premium per member per household (maximum Annual Coverage 3000 RMB) Co pay at a local clinic (n/a) Co-pay at a local hospital Co-pay at a private hospital Deductible at a public facility followed by 30% co-pay (e.g. Peoples (General) Hospital) Deductible at a private facility followed b\ a 30% copax (e.g. Guangminu IAC Hospital) Rural Urban 10 RMB 40-60 RMB 50% 60% 70% 700 RMB 300 RMB A review of the program conducted and reported by the World Bank found that enrollment is lower in poorer households and that the scheme has had no impact on out-of-pocket spending or on utilization among the poor (Wagstaff A et al., 2007) although it has improved access for the poor in urban areas (Liu GG, Zhao Z, Cai R, Yamada T, & Yamada T, 2002). Wagstaff et al's review also found that coverage is mainly limited to in-patient, curative care with very high deductible requirements. The deductible requirement, as illustrated in Table 1, is not trivial. For cataract surgery in Yangjiang City, patients with insurance still have to pay 700RMB for surgery at the People's Hospital. This level of deductible, which must be paid for out-of-pocket is regressive and unfavorable for equity, resulting in either household resource depletion or continued visual impairment. 9