National Health and Nutrition Examination Survey
NHANES at a Glance
| National Health and Nutrition Examination Survey | |
|---|---|
| Data Type | National self-report and examination survey |
| Sample | Nationally representative sample |
| VEHSS Topics Included | Visual Function Vision Exam Measures Eye Health Conditions Service Utilization Examination Measures |
| Years Analyzed | 1999–2008 |
| Approximate Size | 5,000 people per year |
The National Health and Nutrition Examination Survey (NHANES) is a program of studies designed to assess the health and nutritional status of adults and children in the United States. The survey is unique in that it combines interviews and physical examinations. NHANES is a major program of the National Center for Health Statistics (NCHS). NCHS is part of the Centers for Disease Control and Prevention (CDC) and has the responsibility for producing vital and health statistics for the nation. In 1999, the survey became a continuous program that has a changing focus on a variety of health and nutrition measurements to meet emerging needs.
Sample Design
The survey examines a nationally representative sample of about 5,000 people per round, with 2-year data sets available for analysis. These people are located in counties across the country, 15 of which are visited each year. The sample for the survey is selected to represent the US population of all ages. To produce reliable statistics, NHANES over-samples people aged 60 years and older, African Americans, Hispanics, and low-income non-Hispanic White and Other Race adults. In 1999–2006, NHANES oversampled Mexican-Americans, African Americans, low-income White and Other Race people, adolescents aged 12-19, and non-Hispanic White and Other Race adults aged 70 and over. A supplemental sample included pregnant women. In 2007–2008, NHANES oversampled Hispanics, non-Hispanic African Americans, low-income non-Hispanic White and Other Race adults, and non-Hispanic White and Other Race adults aged 80 and over. NHANES generally top-codes age at 85 years, but top-coded at age 80 years in the 2007-2008 data. We therefore top-code all NHANES data at age 80.
Data Collection Procedures
The NHANES interview includes demographic, socioeconomic, dietary, and health-related questions. The examination component consists of medical, dental, and physiological measurements, as well as laboratory tests administered by highly trained medical personnel.
All participants visit the physician. Dietary interviews and body measurements are included for everyone. All but the very young have a blood sample taken and have a dental screening. Depending on the age of the participant, the rest of the examination includes tests to assess various aspects of health. In general, the older the individual, the more extensive the examination.
Health interviews are conducted in respondents’ homes. Health measurements are performed in specially-designed and equipped mobile centers, which travel to locations throughout the country. The study team consists of a physician, medical and health technicians, and dietary and health interviewers. Many of the study staff are bilingual (English/Spanish).
NHANES included visual examinations from 1999–2008. From 2005–2008 NHANES included ophthalmology examinations, including retinal imaging and Frequency Doubling Technology (FDT) tests for visual field loss from eye diseases.
For the VEHSS team, NORC estimated the prevalence rate and sample size for each survey instrument selected for inclusion. We merged samples from the 1999–2008 and 2005–2008 rounds for analysis to maximize the available sample sizes for more detailed levels of stratification.
For binary response questions included in the analysis, prevalence rate was defined as the number of people who gave an affirmative response to the question divided by the total number of respondents who gave an affirmative or negative response. The result was then multiplied by 100 to obtain a percentage. For scaled responses (e.g., mild, moderate, severe), the data value was the proportion of respondents that selected one of the possible response options, and all responses should sum to 100%. We estimated upper and lower confidence intervals and the relative standard error (RSE) of the prevalence estimate. Results in this report calculate confidence intervals using the Clopper-Pearson method.[1] Initially, the cross-tabulated data in the VEHSS data visualization application calculates confidence intervals using standard Wald methods. We plan to change to the Clopper-Pearson method in 2019. Estimates that were based on a sample size less than 30 and/or with a relative standard error greater than 30% were suppressed.
A detailed description of the analytical steps is described in the report “VEHSS Survey Analysis Plan pdf icon[PDF – 480 KB]external icon.” Full analysis documentation is included in the “VEHSS NHANES Data Report pdf icon[PDF – 753 KB]external icon.”
NHANES contains several vision-related questions and examinations relevant to VEHSS. Table 1 includes the NHANES self-report questions analyzed in VEHSS, while Table 2 includes NHANES examination measures calculated by VEHSS.
Table 1. Overview of NHANES Self-Report Variables Included in VEHSS
| VEHSS Indicator Topic | VEHSS Indicator Category | NHANES Variable Name | Years Available (Analyzed) |
Question | Response Options |
|---|---|---|---|---|---|
| Visual Function | Blind or Difficulty Seeing | VIQ017 | 2005–2008 (2005–2008) |
Are you/Is survey participant blind in both eyes? |
|
| Service Utilization | Cataract Surgery | VIQ071 | 1999–2008 (2005–2008) |
Have you/Has survey participant ever had a cataract operation? |
|
| Eye Health Conditions | Self-report glaucoma | VIQ090 | 2005-2008 (2005–2008) | Have you/Has survey participant ever been told by an eye doctor that {you have/s/he has} glaucoma (gla-co-ma), sometimes called high pressure in {your/his/her} eyes? |
|
| Eye Health Conditions | Self-report age-related macular degeneration | VIQ310 | 2005–2008 (2005–2008) | Have you/Has survey participant ever been told by an eye doctor that {you have/s/he has} age-related macular (mac-u-lar) degeneration? |
|
| Eye Health Conditions | Self-report diabetic retinopathy | DIQ080 | 2005–2008 (2005–2008) | Has a doctor ever told you/survey participant that diabetes has affected {your/his/her} eyes or that {you/s/he} had retinopathy (ret-in-op-ath-ee)? |
|
Table 2. Overview of NHANES Examination Variables Included in VEHSS
| VEHSS Indicator Topic | VEHSS Indicator Category | NHANES Variables Used | Years Available (Analyzed) |
|---|---|---|---|
| Eye Health Conditions | Exam-based glaucoma | OPASCST2 – Exam status; OPXDGLAU – Glaucoma, right eye; OPXSGLAU – Glaucoma, left eye |
2005–2008 (2005–2008) |
| Eye Health Conditions | Exam-based age related macular degeneration | OPDUARMA – Any retinopathy, worse eye | 2005–2008 (2005–2008) |
| Eye Health Conditions | Exam-based diabetic retinopathy | DIQ010 – Doctor told you have diabetes; LBXGH – Glycohemoglobin (%); OPDURET – Retinopathy level, worse eye |
2005–2008 (2005–2008) |
| Visual Acuity Measures | Presenting Visual Acuity | VIDRVA – Right visual acuity, presenting; VIDLVA – Left visual acuity, presenting |
1999–2008 (1999–2008) |
| Visual Acuity Measures | Best-corrected Visual Acuity | VIDROVA – Right visual acuity, w/ obj. refraction; VIDLOVA – Left visual acuity, w/ obj. refraction |
1999–2008 (1999–2008) |
| Visual Acuity Measures | Uncorrected Refractive Error | Presenting visual acuity ≤20/50 Best-corrected visual acuity <20/40 with refraction |
1999–2008 (1999–2008) |
This analysis is limited in a number of ways. NHANES does not include state-level data, and NHANES stopped collecting vision and eye health data in 2008. Ophthalmologic exams were only conducted in two two-year cycles, from 2005–2008. NHANES includes approximately 5,000 respondents per year, which means that the available samples are smaller than other surveys included in VEHSS. Additionally, the NHANES sample excludes group quarters including nursing facilities, which may contain a disproportionate share of persons with vision problems. The advantage of NHANES is the presence of examination data, but the exams were limited and did not employ gold-standard dilated examinations.
Additional information about NHANES can be found on the CDC website.