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Original Contribution |

Prevalence of Visual Impairment in the United States FREE

Susan Vitale, PhD, MHS; Mary Frances Cotch, PhD; Robert D. Sperduto, MD
[+] Author Affiliations

Author Affiliations: Division of Epidemiology and Clinical Research, National Eye Institute, National Institutes of Health, Bethesda, Md.

More Author Information
JAMA. 2006;295(18):2158-2163. doi:10.1001/jama.295.18.2158.
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Published online

Context The prevalence of visual impairment in the US public has not been surveyed nationally in several decades.

Objective To estimate the number of US individuals aged 12 years or older who have impaired distance vision due to uncorrected refractive error.

Design, Setting, and Participants The National Health and Nutrition Examination Survey (NHANES), using a multistage probability sampling design, included a vision evaluation in a mobile examination center. Visual acuity data were obtained from 13 265 of 14 203 participants (93.4%) who visited the mobile examination center in 1999-2002. Visual impairment was defined as presenting distance visual acuity of 20/50 or worse in the better-seeing eye. Visual impairment due to uncorrected refractive error was defined as (presenting) visual impairment that improved, aided by automated refraction results, to 20/40 or better in the better-seeing eye.

Main Outcome Measures Presenting distance visual acuity (measured with usual corrective lenses, if any) and distance visual acuity after automated refraction.

Results Overall, 1190 study participants had visual impairment (weighted prevalence, 6.4%; 95% confidence interval [CI], 6.0%-6.8%), and of these, 83.3% could achieve good visual acuity with correction (95% CI, 80.9%-85.8%). Extrapolating these findings to the general US population, approximately 14 million individuals aged 12 years or older have visual impairment (defined as distance visual acuity of 20/50 or worse), and of these, more than 11 million individuals could have their vision improved to 20/40 or better with refractive correction.

Conclusions Visual impairment due to uncorrected refractive error is a common condition in the United States. Providing appropriate refractive correction to those individuals whose vision can be improved is an important public health endeavor with implications for safety and quality of life.

Figures in this Article

Sight is an important indicator of health and quality of life.1,2 Individuals with impaired vision may be at increased risk of injuries3,4 and are more likely to curtail or stop driving.5,6 Visually impaired elderly persons are at increased risk of falls and fractures710 and depression.11 The most common reason for impaired vision may be refractive error,12,13 a condition usually correctable with eyeglasses or contact lenses.

Vision was assessed in the First National Health and Nutrition Examination Survey (NHANES I) (1974-1975)14 and in the Hispanic Health and Nutrition Examination Survey (1982-1984).15 Since then, the US population has aged and become more racially and ethnically diverse16 and rates of myopia (nearsightedness) have increased worldwide.17 Accordingly, visual acuity assessment was reinstituted as an NHANES component in 1999. We estimated the number of US individuals with impaired vision aged 12 years or older and the proportion of individuals who can achieve good distance vision with refractive correction.

Data from NHANES 1999-2002 were used.18 NHANES is a nationally representative survey of the US civilian population, conducted on an ongoing basis by the National Center for Health Statistics, Centers for Disease Control and Prevention. Each year, approximately 7000 individuals are interviewed in their homes and are invited to undergo a comprehensive health examination in a mobile examination center (MEC), comprising functional assessments, collection of biological samples, and laboratory test results.18 Ethnicity is assessed in NHANES to provide estimates of health conditions within specific ethnic groups. The NHANES 1999-2002 protocol was reviewed and approved by the National Center for Health Statistics research ethics review board. Written informed consent was obtained from all participants.

Survey participants provided data on age, sex, ethnicity, years of schooling, income, and health insurance coverage,16 using response options defined by the NHANES investigators. Visual acuity and automated refraction were measured for each eye using the ARK-760 (Nidek Co Ltd, Tokyo, Japan), an autorefractor containing built-in visual acuity charts with 20/20, 20/25, 20/30, 20/40, 20/50, 20/60, 20/80, and 20/200 lines.

Presenting visual acuity was assessed using the participant's usual distance vision correction, if any. The 20/50 line was presented first. At least 4 of the 5 characters (numbers, letters, or both) had to be read correctly to advance to the next line (with smaller characters). If the participant was unable to read the 20/50 line, the 20/200 line was presented. Participants were asked to read each line until they missed more than 1 character per line, for 2 lines in a row. Visual acuity was recorded as the last line for which 4 or more characters were read correctly. For eyes with presenting visual acuity of 20/30 or worse, visual acuity was also measured aided by the automated refraction result (corrected visual acuity). Simple (character recognition) literacy was required for visual acuity testing. Visual acuity was not tested in participants who were too cognitively or physically impaired to undertake the vision examination.19 The autorefractor was calibrated weekly.

Visual impairment was defined as presenting distance visual acuity of 20/50 or worse in the better-seeing eye. We defined visual impairment due to uncorrected refractive error as (presenting) visual impairment that improved, aided by the automated refraction results, to 20/40 or better in the better-seeing eye. Persons with missing presenting or corrected visual acuity for either eye were excluded from analyses (unless 1 eye had presenting visual acuity of 20/40 or better, in which case the participant was classified as not having visual impairment). Participants who did not bring their distance corrective lenses (eyeglasses or contact lenses) to the MEC were classified as not using distance correction.

SUDAAN version 9.0.0 (Research Triangle Institute, Research Triangle Park, NC) was used to compute weighted prevalence estimates accounting for the multistage probability sampling design and nonresponse to the MEC examination.2022 Estimates were age-standardized to the US 2000 Census population.23

A total of 15 193 individuals aged 12 years or older participated in NHANES 1999-2002. Of these participants, 14 203 (93.5%) reported to the MEC and 13 265 (93.4%) had complete visual acuity data (Figure). Those participants with incomplete visual acuity data were more likely to be older, black, poor, and have fewer years of education, and were less likely to have private health insurance compared with those with complete visual acuity data (Table 1).

Figure. Number of NHANES Participants With Complete Visual Acuity Data by Age Group
Graphic Jump Location

NHANES indicates National Health and Nutrition Examination Survey; MEC, mobile examination center. Participants in the incomplete visual acuity groups included persons with incomplete visual acuity data or those whose vision could not be tested because of lack of time, lack of cooperation, cognitive or physical impairment, lack of simple (character recognition) literacy, or equipment failure.

Table Graphic Jump LocationTable 1. Comparison of Characteristics of NHANES Participants With and Without Complete Visual Acuity Data*

Overall, 1190 NHANES participants presented with visual impairment (weighted prevalence, 6.4%; 95% confidence interval [CI], 6.0%-6.8%) (Table 2). Rates of visual impairment were highest among persons who were Hispanic or other (includes Asian or mixed race) ethnicity, were poor, had diabetes mellitus, lacked private health insurance, or had fewer years of education. The majority (83.3%; 95% CI, 80.9%-85.8%) of the 1190 persons with visual impairment could achieve good corrected visual acuity (20/40 or better in the better-seeing eye) using autorefractor correction. The prevalence of visual impairment due to uncorrected refractive error in the total NHANES population (n = 13 265) was 5.3% (95% CI, 4.9%-5.7%).

Table Graphic Jump LocationTable 2. Prevalence of Visual Impairment and Visual Impairment Due to Uncorrected Refractive Error*

Extrapolating the NHANES findings to the general US population, approximately 14 million persons aged 12 years or older would be projected to have visual impairment (Table 3). Of these individuals, more than 11 million could achieve good visual acuity with refractive correction. Of the estimated 2 million persons with presenting visual acuity of 20/200 or worse in the better-seeing eye, 1.4 million could achieve good vision with refractive correction.

Table Graphic Jump LocationTable 3. Total Projected Number of US Individuals (≥12 Years) Who Are Visually Impaired and Need Refractive Correction to Achieve Good Vision*

National surveys, such as the NHANES, provide data to track the health status of the US population for public health planning and policy purposes.24 We estimated that 93.6% of individuals aged 12 years or older have good presenting visual acuity (20/40 or better in the better-seeing eye). Of the remaining 6.4% of persons with visual impairment, the vast majority, 83%, could achieve good visual acuity with refractive correction.

Our results are consistent with those of population-based studies conducted in specific US communities.13,25 Other countries with demographic and socioeconomic profiles similar to the United States have also reported a high proportion of visual impairment due to uncorrected refractive error.2628 Consistent with previous studies,13,25,27,28 we defined visual impairment as visual acuity of 20/50 or worse, and good vision as visual acuity of 20/40 or better. Although 20/40 is the visual acuity criterion used to determine eligibility for a driver's license in most states, we are reluctant to call this normal vision.

We found that prevalence of visual impairment was higher in persons who were of black, Hispanic, or other ethnicity, or who were poor, less educated, or lacked private health insurance. Although these findings are not unexpected, they suggest that health care access and resources may be important barriers to consider in addressing the need for refractive correction of visual impairment.

Our analyses are subject to several limitations. The MEC vision examination was conducted by trained operators using standardized protocols under stringent quality control procedures.19 Nevertheless, visual acuity measured by an autorefractor is not equivalent29 to visual acuity assessed in a clinical ophthalmic setting (precluded by logistical constraints in NHANES). We are unaware of studies validating the visual acuity measured using autorefractor charts and are therefore unable to address the accuracy of the NHANES measurements. Although similar autorefractors have been shown to be valid and reliable,30,31 the automated refraction obtained in NHANES is not equivalent to a subjectively refined refraction. Our corrected visual acuity is therefore not comparable with best-corrected visual acuity used in other studies. Despite these methodological limitations, our estimates are similar to results from previous epidemiological studies of visual impairment due to refractive error.

Our results may be influenced by nonparticipation in the vision examination. We followed the NHANES analytic guidelines to adjust for nonresponse (to the MEC examination) by using sampling weights.20 However, our results indicate that persons without visual acuity data were more likely to be poor, less educated, and of nonwhite ethnicity. These characteristics were also associated with higher rates of visual impairment in our sample; therefore, prevalence of visual impairment may be underestimated in our study.

We elected to include in our analysis the 765 persons (5.8% of the total sample) who informed NHANES personnel they had refractive correction for distance but forgot to bring their lenses to the MEC; 29% of these persons were found to need refractive correction to have visual acuity of 20/40 or better. If these individuals were excluded, prevalence of visual impairment would decrease to 5.1% (vs 6.4%) and prevalence of visual impairment due to refractive error would decrease to 4.1% (vs 5.3%). Although we think it is appropriate to include these individuals in estimates of visual impairment based on presenting visual acuity, their inclusion may result in an overestimate of the prevalence of visual impairment due to refractive error.

In conclusion, the results of this study suggest that approximately 14 million individuals aged 12 years or older were estimated to be visually impaired. More than 11 million of these individuals could attain visual acuity good enough to qualify them for a driver's license in most states with refractive correction. The provision of corrective lenses to those individuals in need will be an important public health endeavor with implications for safety and quality of life.

Corresponding Author: Susan Vitale, PhD, MHS, Division of Epidemiology and Clinical Research, National Eye Institute, National Institutes of Health, 5635 Fishers Ln, #1100, MSC 9301, Bethesda, MD 20892 (sev@nei.nih.gov).

Author Contributions: Dr Vitale had full access to all of the data in the study and takes responsibility for the integrity of the data and the accuracy of the data analysis.

Study concept and design: Vitale, Cotch, Sperduto.

Acquisition of data: Vitale, Cotch.

Analysis and interpretation of data: Vitale, Cotch, Sperduto.

Drafting of the manuscript: Vitale, Cotch.

Critical revision of the manuscript for important intellectual content: Vitale, Cotch, Sperduto.

Statistical analysis: Vitale.

Obtained funding: Cotch, Sperduto.

Administrative, technical, or material support: Cotch, Sperduto.

Financial Disclosures: None reported.

Funding/Support: This study was supported by the National Center for Health Statistics (NCHS), Centers for Disease Control and Prevention. Additional funding for the National Health and Nutrition Examination Survey (NHANES) Vision Component was provided by Intramural Research Program grant Z01EY000402 from the National Eye Institute, National Institutes of Health. The NCHS has contracted with Westat Inc to conduct the field operations and data collection for NHANES.

Role of the Sponsor: The NCHS was involved in the design and conduct of the study and in data collection, but was not involved in the analysis or interpretation of the vision examination study results or in the preparation, review, or approval of the manuscript. The National Eye Institute provided funding support for the vision examination and was involved in the design and conduct of the vision component, in the collection, analysis, and interpretation of the vision data, and in the preparation, review, and approval of this article before submission.

Acknowledgment: We thank the staff of the NCHS, Centers for Disease Control and Prevention, especially Brenda G. Lewis, BS, ASCP, MPH, for their expert assistance with all technical aspects of the vision component. Ms Lewis received salary compensation from NCHS. We also thank the staff at Westat Inc, especially Kay Apodaca, BA, MSW, and Beryl D. Carew, MPH, BN, for their role with data collection activities. Mss Apodaca and Carew received salary compensation from their employer, Westat Inc (contracted by NCHS). We thank Susan K. Corwin, BSMA, CO, COMT, Director of Training and Education for Marco. Ms Corwin was paid by Westat Inc, for her role as a consultant in training the NHANES field staff technicians in the use of the Marco/Nidek equipment. We also thank Roy W. Beck, MD, PhD, Jaeb Center for Health Research, for technical advice in designing the NHANES vision component protocol. Dr Beck did not receive compensation for his role in the study. We are grateful to all the people who agreed to participate in NHANES, without whose time and dedication this study would not have been possible.

Broman AT, Muñoz B, Rodriguez J.  et al.  The impact of visual impairment and eye disease on vision-related quality of life in a Mexican-American population: Proyecto VER.  Invest Ophthalmol Vis Sci. 2002;43:3393-3398
PubMed
Swanson MW, McGwin G. Visual impairment and functional status from the 1995 National Health Interview Survey on Disability.  Ophthalmic Epidemiol. 2004;11:227-239
PubMed   |  Link to Article
Zwerling C, Whitten PS, Davis CS.  et al.  Occupational injuries among older workers with visual, auditory, and other impairments: a validation study.  J Occup Environ Med. 1998;40:720-723
PubMed   |  Link to Article
Roberts I, Norton R. Sensory deficit and the risk of pedestrian injury.  Inj Prev. 1995;1:12-14
PubMed   |  Link to Article
Keeffe JE, Jin DF, Weih LM, McCarty CA, Taylor HR. Vision impairment and older drivers: who's driving?  Br J Ophthalmol. 2002;86:1118-1121
PubMed   |  Link to Article
Owsley C, McGwin G Jr. Vision impairment and driving.  Surv Ophthalmol. 1999;43:535-550
PubMed   |  Link to Article
Dargent-Molina P, Favier F, Grandjean H.  et al.  Fall-related factors and risk of hip fracture: the EPIDOS prospective study.  Lancet. 1996;348:145-149
PubMed   |  Link to Article
Felson DT, Anderson JJ, Hannan MT, Milton RC, Wilson PW, Kiel DP. Impaired vision and hip fracture: the Framingham Study.  J Am Geriatr Soc. 1989;37:495-500
PubMed
Ivers RQ, Norton R, Cumming RG, Butler M, Campbell AJ. Visual impairment and risk of hip fracture.  Am J Epidemiol. 2000;152:633-639
PubMed   |  Link to Article
Ivers RQ, Cumming RG, Mitchell P, Attebo K. Visual impairment and falls in older adults: the Blue Mountains Eye Study.  J Am Geriatr Soc. 1998;46:58-64
PubMed
Tsai SY, Cheng CY, Hsu WM, Su TP, Liu JH, Chou P. Association between visual impairment and depression in the elderly.  J Formos Med Assoc. 2003;102:86-90
PubMed
Dimitrov PN, Mukesh BN, McCarty CA, Taylor HR. Five-year incidence of bilateral cause-specific visual impairment in the Melbourne Visual Impairment Project.  Invest Ophthalmol Vis Sci. 2003;44:5075-5081
PubMed   |  Link to Article
Muñoz B, West SK, Rubin GS.  et al.  Causes of blindness and visual impairment in a population of older Americans: the Salisbury Eye Evaluation Study.  Arch Ophthalmol. 2000;118:819-825
PubMed   |  Link to Article
Engel A, Murphy RS, Maurer K, Collins E. Plan and operation of the HANES I augmentation survey of adults 25-74 years United States, 1974-1975.  Vital Health Stat 1. 1978;((14)):1-110
PubMed
 Plan and operation of the Hispanic Health and Nutrition Examination Survey, 1982-84.  Vital Health Stat 1. 1985;((19)):1-429
PubMed
National Center for Health Statistics.  Health, United States, 2004, Chartbook on Trends in the Health of Americans. Washington, DC: US Dept of Health and Human Services; 2004. DHHS publication 2004-1232
Rose K, Smith W, Morgan I, Mitchell P. The increasing prevalence of myopia: implications for Australia.  Clin Exp Ophthalmol. 2001;29:116-120
PubMed   |  Link to Article
National Health and Nutrition Examination Survey.  Public-Use Data Files. http://www.cdc.gov/nchs/about/major/nhanes/datalink.htm. Accessed February 16, 2005
National Health and Nutrition Examination Survey.  Vision Procedures Manual. http://www.cdc.gov/nchs/data/nhanes/nhanes_01_02/vision_year_3.pdf. Accessed March 14, 2006
 NHANES Analytic Guidelines, June 2004 Version. Hyattsville, Md: National Center for Health Statistics, US Centers for Disease Control and Prevention; 2004. http://www.cdc.gov/nchs/data/nhanes/nhanes_general_guidelines_june_04.pdf. Accessed February 16, 2006
 Analytic and Reporting Guidelines: The Third National Health and Nutrition Examination Survey, NHANES III (1988-94). Hyattsville, Md: National Center for Health Statistics, US Centers for Disease Control and Prevention; 1996. http://www.cdc.gov/nchs/data/nhanes/nhanes3/nh3gui.pdf. Accessed February 16, 2006
 NHANES 1999-2000 Addendum to the NHANES III Analytic Guidelines. Hyattsville, Md: National Center for Health Statistics, US Centers for Disease Control and Prevention; 2002. http://www.cdc.gov/nchs/data/nhanes/guidelines1.pdf. Accessed February 16, 2006
Klein RJ, Schoenborn CA. Age adjustment using the 2000 projected US population. In: Healthy People 2010, Statistical Notes, No. 20. Hyattsville, Md: National Center for Health Statistics, US Centers for Disease Control and Prevention, US Dept of Health and Human Services; 2001
 National Health and Nutrition Examination Survey: Background. US Centers for Disease Control and Prevention Web site. http://www.cdc.gov/nchs/about/major/nhanes/bhistory.htm. Accessed December 6, 2005
Muñoz B, West SK, Rodriguez J.  et al.  Blindness, visual impairment and the problem of uncorrected refractive error in a Mexican-American population: Proyecto VER.  Invest Ophthalmol Vis Sci. 2002;43:608-614
PubMed
Evans BJ, Rowlands G. Correctable visual impairment in older people: a major unmet need.  Ophthalmic Physiol Opt. 2004;24:161-180
PubMed   |  Link to Article
Foran S, Rose K, Wang JJ, Mitchell P. Correctable visual impairment in and older population: the Blue Mountains Eye Study.  Am J Ophthalmol. 2002;134:712-719
PubMed   |  Link to Article
Thiagalingam S, Cumming RG, Mitchell P. Factors associated with undercorrected refractive errors in an older population: the Blue Mountains Eye Study.  Br J Ophthalmol. 2002;86:1041-1045
PubMed   |  Link to Article
Heron G, Furby HP, Walker RJ, Lane CS, Judge OJ. Relationship between visual acuity and observation distance.  Ophthalmic Physiol Opt. 1995;15:23-30
PubMed   |  Link to Article
Pesudovs K. Autorefraction as an outcome measure of laser in situ keratomileusis.  J Cataract Refract Surg. 2004;30:1921-1928
PubMed   |  Link to Article
Allen PM, Radhakrishnan H, O’Leary DJ. Repeatability and validity of the PowerRefractor and the Nidek AR600-A in an adult population with healthy eyes.  Optom Vis Sci. 2003;80:245-251
PubMed   |  Link to Article

Figures

Figure. Number of NHANES Participants With Complete Visual Acuity Data by Age Group
Graphic Jump Location

NHANES indicates National Health and Nutrition Examination Survey; MEC, mobile examination center. Participants in the incomplete visual acuity groups included persons with incomplete visual acuity data or those whose vision could not be tested because of lack of time, lack of cooperation, cognitive or physical impairment, lack of simple (character recognition) literacy, or equipment failure.

Tables

Table Graphic Jump LocationTable 1. Comparison of Characteristics of NHANES Participants With and Without Complete Visual Acuity Data*
Table Graphic Jump LocationTable 2. Prevalence of Visual Impairment and Visual Impairment Due to Uncorrected Refractive Error*
Table Graphic Jump LocationTable 3. Total Projected Number of US Individuals (≥12 Years) Who Are Visually Impaired and Need Refractive Correction to Achieve Good Vision*

References

Broman AT, Muñoz B, Rodriguez J.  et al.  The impact of visual impairment and eye disease on vision-related quality of life in a Mexican-American population: Proyecto VER.  Invest Ophthalmol Vis Sci. 2002;43:3393-3398
PubMed
Swanson MW, McGwin G. Visual impairment and functional status from the 1995 National Health Interview Survey on Disability.  Ophthalmic Epidemiol. 2004;11:227-239
PubMed   |  Link to Article
Zwerling C, Whitten PS, Davis CS.  et al.  Occupational injuries among older workers with visual, auditory, and other impairments: a validation study.  J Occup Environ Med. 1998;40:720-723
PubMed   |  Link to Article
Roberts I, Norton R. Sensory deficit and the risk of pedestrian injury.  Inj Prev. 1995;1:12-14
PubMed   |  Link to Article
Keeffe JE, Jin DF, Weih LM, McCarty CA, Taylor HR. Vision impairment and older drivers: who's driving?  Br J Ophthalmol. 2002;86:1118-1121
PubMed   |  Link to Article
Owsley C, McGwin G Jr. Vision impairment and driving.  Surv Ophthalmol. 1999;43:535-550
PubMed   |  Link to Article
Dargent-Molina P, Favier F, Grandjean H.  et al.  Fall-related factors and risk of hip fracture: the EPIDOS prospective study.  Lancet. 1996;348:145-149
PubMed   |  Link to Article
Felson DT, Anderson JJ, Hannan MT, Milton RC, Wilson PW, Kiel DP. Impaired vision and hip fracture: the Framingham Study.  J Am Geriatr Soc. 1989;37:495-500
PubMed
Ivers RQ, Norton R, Cumming RG, Butler M, Campbell AJ. Visual impairment and risk of hip fracture.  Am J Epidemiol. 2000;152:633-639
PubMed   |  Link to Article
Ivers RQ, Cumming RG, Mitchell P, Attebo K. Visual impairment and falls in older adults: the Blue Mountains Eye Study.  J Am Geriatr Soc. 1998;46:58-64
PubMed
Tsai SY, Cheng CY, Hsu WM, Su TP, Liu JH, Chou P. Association between visual impairment and depression in the elderly.  J Formos Med Assoc. 2003;102:86-90
PubMed
Dimitrov PN, Mukesh BN, McCarty CA, Taylor HR. Five-year incidence of bilateral cause-specific visual impairment in the Melbourne Visual Impairment Project.  Invest Ophthalmol Vis Sci. 2003;44:5075-5081
PubMed   |  Link to Article
Muñoz B, West SK, Rubin GS.  et al.  Causes of blindness and visual impairment in a population of older Americans: the Salisbury Eye Evaluation Study.  Arch Ophthalmol. 2000;118:819-825
PubMed   |  Link to Article
Engel A, Murphy RS, Maurer K, Collins E. Plan and operation of the HANES I augmentation survey of adults 25-74 years United States, 1974-1975.  Vital Health Stat 1. 1978;((14)):1-110
PubMed
 Plan and operation of the Hispanic Health and Nutrition Examination Survey, 1982-84.  Vital Health Stat 1. 1985;((19)):1-429
PubMed
National Center for Health Statistics.  Health, United States, 2004, Chartbook on Trends in the Health of Americans. Washington, DC: US Dept of Health and Human Services; 2004. DHHS publication 2004-1232
Rose K, Smith W, Morgan I, Mitchell P. The increasing prevalence of myopia: implications for Australia.  Clin Exp Ophthalmol. 2001;29:116-120
PubMed   |  Link to Article
National Health and Nutrition Examination Survey.  Public-Use Data Files. http://www.cdc.gov/nchs/about/major/nhanes/datalink.htm. Accessed February 16, 2005
National Health and Nutrition Examination Survey.  Vision Procedures Manual. http://www.cdc.gov/nchs/data/nhanes/nhanes_01_02/vision_year_3.pdf. Accessed March 14, 2006
 NHANES Analytic Guidelines, June 2004 Version. Hyattsville, Md: National Center for Health Statistics, US Centers for Disease Control and Prevention; 2004. http://www.cdc.gov/nchs/data/nhanes/nhanes_general_guidelines_june_04.pdf. Accessed February 16, 2006
 Analytic and Reporting Guidelines: The Third National Health and Nutrition Examination Survey, NHANES III (1988-94). Hyattsville, Md: National Center for Health Statistics, US Centers for Disease Control and Prevention; 1996. http://www.cdc.gov/nchs/data/nhanes/nhanes3/nh3gui.pdf. Accessed February 16, 2006
 NHANES 1999-2000 Addendum to the NHANES III Analytic Guidelines. Hyattsville, Md: National Center for Health Statistics, US Centers for Disease Control and Prevention; 2002. http://www.cdc.gov/nchs/data/nhanes/guidelines1.pdf. Accessed February 16, 2006
Klein RJ, Schoenborn CA. Age adjustment using the 2000 projected US population. In: Healthy People 2010, Statistical Notes, No. 20. Hyattsville, Md: National Center for Health Statistics, US Centers for Disease Control and Prevention, US Dept of Health and Human Services; 2001
 National Health and Nutrition Examination Survey: Background. US Centers for Disease Control and Prevention Web site. http://www.cdc.gov/nchs/about/major/nhanes/bhistory.htm. Accessed December 6, 2005
Muñoz B, West SK, Rodriguez J.  et al.  Blindness, visual impairment and the problem of uncorrected refractive error in a Mexican-American population: Proyecto VER.  Invest Ophthalmol Vis Sci. 2002;43:608-614
PubMed
Evans BJ, Rowlands G. Correctable visual impairment in older people: a major unmet need.  Ophthalmic Physiol Opt. 2004;24:161-180
PubMed   |  Link to Article
Foran S, Rose K, Wang JJ, Mitchell P. Correctable visual impairment in and older population: the Blue Mountains Eye Study.  Am J Ophthalmol. 2002;134:712-719
PubMed   |  Link to Article
Thiagalingam S, Cumming RG, Mitchell P. Factors associated with undercorrected refractive errors in an older population: the Blue Mountains Eye Study.  Br J Ophthalmol. 2002;86:1041-1045
PubMed   |  Link to Article
Heron G, Furby HP, Walker RJ, Lane CS, Judge OJ. Relationship between visual acuity and observation distance.  Ophthalmic Physiol Opt. 1995;15:23-30
PubMed   |  Link to Article
Pesudovs K. Autorefraction as an outcome measure of laser in situ keratomileusis.  J Cataract Refract Surg. 2004;30:1921-1928
PubMed   |  Link to Article
Allen PM, Radhakrishnan H, O’Leary DJ. Repeatability and validity of the PowerRefractor and the Nidek AR600-A in an adult population with healthy eyes.  Optom Vis Sci. 2003;80:245-251
PubMed   |  Link to Article

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