Sample Design 2015-2018

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National Health and Nutrition Examination Survey

Sample Design 2015-2018

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Attachment 14b - Current NHANES 2015-2018 sample design approved by the NCHS Research Ethics Review Board

III. Study Design

A. Sample Design and Study Population


The sample design of NHANES is based on a continuous on-going annual survey of the non-institutionalized, civilian population of the U.S. Each single year and any combination of consecutive years will comprise a nationally representative sample of the United States population. Data are publicly released every two years to improve analytic stability of estimates and to decrease disclosure risk.


Several subgroups are oversampled to ensure adequate sample sizes for selected population subgroups. For NHANES 2017-2018, African Americans, Hispanics, Asian Americans, older persons, and low-income whites will be oversampled. Table 1 shows the sampling domains for NHANES. These domains represent the analytic subpopulations of interest. For NHANES 2015-2018, there are 87 sampling domains defined by race and Hispanic origin, gender, age, and, for the white/other domains, low income status (i.e., households in which the household income is below 185% of the poverty level).


The sample design for NHANES is planned for four years. The design for NHANES 2015-2018 was planned in 2012 and 2013. After the sampled primary sampling units (PSUs) were selected, no change is made during those four years. Table 1 shows the annual and cumulative estimates of the target sample sizes by analytic subdomain, based on the assumption that two mobile examination center (MEC) teams will be in operation and approximately 5,000 persons will be examined in 15 PSUs per year. The expected sample size is based on past NHANES experience with response rates for each subdomain of interest. The goal for the overall examination response rate for NHANES 2015-2018 was targeted for 70 percent. In NHANES 2014, 2015, and 2016 the examination response rates were 65, 63 and 55 percent, respectively. Experiences prior to 2014 indicated a sample design based on a target of 70 percent was reasonable. Additionally, we have a goal of performing 5,000 examinations per year. When a lower than expected response has occurred in PSUs, more screening is performed to obtain more examinations in an attempt to meet the goal of 5,000 examinations. The ability to screen more is limited by available time in a PSU and available field staff. We are currently determining the NHANES 2019-2020 sample design; and the projected response rates will be more consistent with recent trends.


B. Sampling Methods

As with previous NHANES surveys, the design for the current NHANES is a stratified, multistage probability sample of the civilian non-institutionalized population of the U.S. In hierarchical order, the stages of the sample selection are first: selection of Primary Sampling Units or PSUs (counties or small groups of contiguous counties); second: segments within PSU (a block or group of blocks containing a cluster of households); third: households within segments; and fourth: one or more survey participants within households.


To produce unbiased cross-sectional estimates for the entire civilian, non-institutionalized population of the United States, the sample data is inflated to the level of the population from which the sample is drawn. As in previous NHANES, the sampling weight for each sample person is the product of three factors: the reciprocal of the probabilities of selection (PSU, segment, household, person); an adjustment for non-response; and a post-stratification adjustment factor to make the resulting survey estimates in each age-sex-race-Hispanic origin income icategory approximately equal to independent control totals from the American Community Survey (ACS) conducted by the U.S. Bureau of the Census.


To analyze multiple year samples, sampling weights can either be averaged over the sampled years used or can be readjusted to population controls for the midpoint of the combined years. Variances for NHANES are estimated using the Taylor series linearization procedure, SUDAAN, available through the Statistical Analysis System. This technique has been used in previous NHANES and the Hispanic HANES (HHANES). Alternatively, jackknife and BRR-type replicate weights are generated to allow other replication methods to compute variance when sample sizes may not be adequate to produce reliable estimates from Taylor's series approximations.


A primary objective of NHANES is to estimate, with acceptable precision, the health and nutritional status of subgroups of the population. For NHANES 2015-2018, the analytic subgroups of interest are defined through 87 sampling domains (sampling flags) by race/Hispanic origin, gender, age, and low income status (i.e., households in which the household income is below 185% of the poverty level) for the white/other domains. These domains can be found in Table 1. A number of domains require oversampling to ensure adequate sample sizes are achieved so selected health conditions can be estimated for minority groups. The over-sampled groups include African Americans, Hispanics, Asian Americans, younger and older persons, and low-income white/other domains. The sampling procedure is implemented using a set of sampling flags that designate for each household which domains are eligible for sampling. This is accomplished by selecting a sample of households with equal probability at a rate equal to the maximum oversampling rate assigned to the domains. Each selected household is then assigned a set of sampling flags corresponding to the 87 sampling domains. The flags are set at the domain sampling rates assigned to achieve the target sample size for each domain. The sampling flags are then pre-loaded into the Computer Assisted Personal Interview (CAPI) system on NHANES laptops prior to the start of data collection. That is, interviewers are not required to carry out any subsampling operation. They are instead instructed by the system on which persons to include in the sample.


Therefore, the probability of selection is related to race/Hispanic origin, gender, age, and low-income status. This differential probability of selection is designed to meet departmental and public health objectives to reduce disparities. The NHANES sample persons, however, are chosen at random.

Table 1: Projected population size, number of sampled persons, and projected response rates for NHANES 2015-2018 in 60 PSUs by age, race and Hispanic origin, income, and gender

 

 

 

Projected population average over years 2015-20181

Total sample

Estimated exam response rate2

Target number of exams for 2015-2018








Black, non-Hispanic

M&F

< 1 year

726,585

216

85%

184



1-2 yrs.

1,439,050

370

83%

308



3-5 yrs.

2,092,808

387

80%

308


M

6-11 yrs.

2,010,582

383

80%

308



12-19 yrs.

2,557,093

390

79%

308



20-39 yrs.

5,531,423

665

74%

492



40-49 yrs.

2,247,656

343

71%

244



50-59 yrs.

2,379,097

351

69%

244



60+ yrs.

2,758,652

804

63%

504


F

6-11 yrs.

1,953,238

384

80%

308



12-19 yrs.

2,544,785

401

77%

308



20-39 yrs.

6,421,630

656

75%

492



40-49 yrs.

2,752,614

339

72%

244



50-59 yrs.

2,848,545

327

75%

244

 

 

60+ yrs.

3,836,365

788

64%

504

Total Black, non-Hispanic

42,100,123

6,806

73%

5,000








Hispanic

M&F

<1 year

1,171,535

334

86%

288



1-2 yrs.

2,316,621

333

87%

288



3-5 yrs.

3,362,637

353

82%

288


M

6-11 yrs.

3,225,375

338

86%

292



12-19 yrs.

3,943,976

356

82%

292



20-39 yrs.

9,528,110

720

68%

492



40-49 yrs.

3,804,496

330

75%

248



50-59 yrs.

2,827,360

364

68%

248



60+ yrs.

2,659,160

814

61%

496


F

6-11 yrs.

3,086,758

357

82%

292



12-19 yrs.

3,783,981

371

79%

292



20-39 yrs.

8,777,249

613

80%

492



40-49 yrs.

3,721,308

324

76%

248



50-59 yrs.

2,865,586

339

73%

248

 

 

60+ yrs.

3,259,775

881

56%

496

Total Hispanic



58,333,927

6,828

73%

5,000








Non-Hispanic,

M&F

< 1 year

273,292

79

66%

52

non-Black Asian


1-2 yrs.

546,059

162

62%

100



3-5 yrs.

804,702

241

66%

160


M

6-11 yrs.

784,302

237

61%

144



12-19 yrs.

978,909

292

64%

188



20-39 yrs.

2,868,204

554

58%

324



40-49 yrs.

1,325,307

326

50%

164



50-59 yrs.

1,060,302

319

50%

160



60+ yrs.

1,318,080

397

46%

184


F

6-11 yrs.

758,637

223

50%

112



12-19 yrs.

960,905

288

61%

176



20-39 yrs.

3,087,674

571

55%

316



40-49 yrs.

1,519,346

314

51%

160



50-59 yrs.

1,226,492

287

56%

160

 

 

60+ yrs.

1,707,959

508

39%

200

Total non-Hispanic, non-Black Asian

19,220,170

4,800

54%

2,600








Non-Hispanic White/Other

M&F

< 1 year

475,509

140

97%

136

Low Income


1-2 yrs.

922,061

169

81%

136



3-5 yrs.

1,346,124

154

91%

140


M

6-11 yrs.

1,216,699

150

93%

140



12-19 yrs.

1,469,297

151

93%

140



20-29 yrs.

2,120,036

141

76%

108



30-39 yrs.

1,384,933

127

85%

108



40-49 yrs.

1,253,827

128

84%

108



50-59 yrs.

1,492,999

149

70%

104



60-69 yrs.

1,257,791

146

71%

104



70-79 yrs.

628,636

162

64%

104



80+ yrs.

401,776

122

52%

64


F

6-11 yrs.

1,144,717

158

89%

140



12-19 yrs.

1,467,306

171

82%

140



20-29 yrs.

3,015,976

123

84%

104



30-39 yrs.

1,956,563

117

89%

104



40-49 yrs.

1,457,591

115

90%

104



50-59 yrs.

1,841,567

124

84%

104



60-69 yrs.

1,690,869

145

72%

104



70-79 yrs.

1,354,362

130

80%

104

 

 

80+ yrs.

1,273,741

203

51%

104

Total non-Hispanic White/Other Low Income

29,172,380

3,028

79%

2,400








Non-Hispanic White/Other

M&F

< 1 year

1,657,990

395

73%

288

Not Low Income


1-2 yrs.

3,358,271

519

56%

288



3-5 yrs.

5,020,468

449

64%

288


M

6-11 yrs.

5,336,202

423

69%

292



12-19 yrs.

7,829,377

413

71%

292



20-29 yrs.

10,253,435

351

60%

212



30-39 yrs.

10,708,541

361

59%

212



40-49 yrs.

10,911,032

346

61%

212



50-59 yrs.

13,237,738

364

58%

212



60-69 yrs.

11,863,100

357

59%

212



70-79 yrs.

6,916,646

399

53%

212



80+ yrs.

3,292,269

450

47%

212


F

6-11 yrs.

5,089,881

401

73%

292



12-19 yrs.

7,429,312

434

67%

292



20-29 yrs.

9,355,737

388

55%

212



30-39 yrs.

10,317,801

336

63%

212



40-49 yrs.

10,882,958

307

69%

212



50-59 yrs.

13,380,811

348

61%

212



60-69 yrs.

12,395,307

351

60%

212



70-79 yrs.

7,350,344

408

52%

212

 

 

80+ yrs.

4,297,525

513

41%

212

Total non-Hispanic White/Other Not Low Income

170,884,743

8,312

60%

5,000

Total non-Hispanic White/Other



200,057,123

11,339

65%

7,400








TOTAL



319,711,342

29,773

67%

20,000








NOTES: M is male. F is female. Yrs. is years.

1 Civilian non-institutionalized population in the 50 United States and the District of Columbia.

2 Unconditional response rates are the domain-level response rates (number of examined persons divided by the number of identified sampled persons) from the 2011-2012 experience, each adjusted downward by the overall screener response rate experienced in those years to result in conservative estimates.





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