The survey design
The sampling plan for the first 65 stands, or loca-tions, of the fnst National Health and Nutrition Ex-amination Survey (NHANES I) followed a stratified multistage probability design in which a sample of the civilian noninstit utiomdized population ages 1-74 years of the coterminous United States was selected.
Excluded from the selection were persons residing in Alaska and Hawaii and those within the coterminous United States who were confined to institutions or residing on American Indian reservation lands. Suc-cessive elements dealt with in the process of sampling were the primary sampling unit (PSU), census enu-meration district, segment (a cluster of households), household, eligible pmsons, and finally, sample persons.
The starting points in the first stage of this design were the 1960 decennid census lists of addresses and the nearly 100 PSU’S into which the entire United States was divided by the U.S. Bureau of the Census.
Each PSU is either a st~ndard metropolitan statistical area (SMSA), a single county, or two or three con-tiguous counties. The 1’SU’S were grouped into 357 strata for use in the Health Interview Survey and were then collapsed into 40 superstrata for use in Cycles 11 and 111 of the Health Examination Survey and NHANES I.
Fifteen of the 40 superstrata contained a single large metropolitan are~ with a population of more than 2 million. The 15 large metropolitan areas were selected for the sample with certainty. The 25 non-certainty strata were classified into 4 broad popula-tion density groups in each region. Then a controlled selection technique was used to select 2 PSU’S from each of the 25 noncertainty superstrata with the probability of selection of a PSU proportionate to its
1960 population. Thus, proportionate representation of specified State groups and rate-of-population-change classes was maintained in the sample. In this manner a total first-stage sample of 65 PSU’S was selected. These 65 sample PSU’S, or stands, are the
areas within which a sample of persons would be se-lected for examination over the 3-year survey period.
Although the 1970 census data were used as the frame for selecting the sample of PSU’S when they be-came available, the calendar of operations required that 1960 census data be used for 44 of the 65 stands in the NHANES I sample. Census enumeration dis-tricts (ED’s) in each PSU were divided into segments of an expected six housing units each. In urban ED’s the segments were clusters of six addresses from the 1960 census listing books. For ED’s not having usable addresses, area sampling was employed and, conse-quently, the segment size varied. To make the sample representative of the current population of the United States, the address or list segments were supplemented by a sample of housing units that had been con-structed since 1960.
Within each PSU a systematic sample of segments was selected. The ED’s that fell into the sample were coded into one of two economic classes. The fust class, identified as the “poverty stratum,” was com-posed of current poverty areas that had been identi-fied by the U.S. Bureau of the Census in 1970 (pre-1970 census). plus other ED’s in the PSU with a mean income of less than $3,000 in 1959 (based on 1960 census data). The second economic class, the “non-poverty stratum, “ included all ED’s not designated as belonging to the poverty stratum.
All sample segments classified as being in the poverty stratum were retained in the sample. For those sample segments in nonpoverty stratum ED’s, the selected segments were divided into eight random subgroups and one of the subgroups was chosen to remain in the NHANES I sample. This procedure per-mits separate analyses with adequate reliability of those classified as being below the poverty level and those classified as being above the poverty level.
After the sample segments had been identified, a list of all current addresses within the segment bound-aries was made. and a person in each of the house-holds was interviewed to determine the age and sex of
each household member and to gather other demo-graphic and socioeconomic information required for the survey.
In selecting the persons in sample segments to be examined in NHANES I, all household members ages 1-74 years in each segment were listed on a sample selection worksheet with each household in the seg-ment listed serially. The number of household mem-bers in each of the six age-sex groups shown below were listed on the worksheet under the appropriate age-sex-group column, The sample selection work-sheets were then ordered by segment number, and a systematic random sample of persons in each age-sex group was selected to be examined by using the following sampling rates:
The persons selected for the 65-stand nutrition component of NHANES I constitute a representative sample of the target population and include 28,043 persons ages 1-74 years, of whom 20,749, or nearly 74 percent, were examined. When adjustments are made for differential sampling for high risk groups, the response rate becomes 75.2 percent.
The subsample of adults ages 25-74 years who re-ceived the detailed examination in addition to the more general nutrition examination was chosen sys-te matically afsys-ter a random start, This group com-prised one-fifth of the total sample of adults ages 25-74 years and was selected in accordance with the following rates:
The continuation of the detailed medical exami-nation from 1974 to 1975 is referred to as the NHANES I augmentation survey of adults. The sam-ple design had two basic requirements: The sample of persons selected for examination in survey loca-tions 66-100 would constitute a national probability sample of the target population, and, when consid-ered jointly with those persons receiving the detailed examination in NHANES I survey locations 1-65, the sample would be a 100-PSU national probability sample, Table I is a list of the sample locations at
which examinations were conducted during the survey.
The PSU’S for the augmentation survey were select ed by the same process used earlier at the begin-ning of NHANES I (described earlier4 ). The final sampling stage involved the random selection of one of every two adults ages 25-74 years who were eligi-ble for the sample.
All the data presented in this report are based on
“weighted” observations. That is, data recorded for each sample person are inflated to characterize the subuniverse from which that sample person was drawn.
Derivation of estimates
Because the design of NHANES I is a multistage probability sample, complex procedures must be used in the derivation of estimates. Three basic operations are involved, the results of which are presented separ-ately in tables II and III for the nutrition examina-tion sample and the detailed examination sample, respectively.
Inflation by the reciprocal of the probability of selection. –The probability of selection is the product
of the probabilities of selection from each step of selection in the design (PSU, segment, and sample person).
Nonresponse adjustment. -The estimates are in-flated by a multiplication factor calculated within each PSU for each of five selected income groups (less than $3,000; $3,000-$6,999; $7,000-$ 9,9!99;
$10,000-$ 14,999; and $15,000 and over). The Iuu-merator of these factors consists of the sum of the weights for sample persons resulting from the recipro-cal of the probability of selection, and the denomina-tor consists of the sum of the weights for examined persons also resulting from the reciprocal of the probability of selection,
Poststratification by age-sex-race. –The estimates are ratio adjusted within age-sex-race cells to an inde-pendent estimate, provided by the U.S. Bureau of the Census, of the population of each cell as of the mlid-point of the survey. The effect of the ratio-estimating process is to make the sample more closely represent-ative of the U.S. civilian noninstitutionalized popula-tion by age, sex, and race, which thereby reduces sampling variance.
More detailed descriptions of the survey design and selection technique have been published.’~’5 Nonresponse
In any health examination survey, after the sample is identified and the sample persons are re-quested to participate in the examination, the survey meets one of its more severe problems: Usually, a sizable number of sample persons will not participate
NOTE: A list of references follows the text.
Table 1. Sample locations of the first National Health and Nutrition Examination Survey by ragion, county, and State
Region, coun W, 7 and state
Northeast
Essex, Morris, Union, Somerset, Hudson, Middlesex, N.J.
Nassau, Queens, Suffolk, N.Y.
Bronx, N.Y.
Kings, Richmond, N.Y.
Westchester, Rockland, N. Y.: Bergen, Passaic, N,J.
Bucks, Chestar, Delaware, Montgome~, Philadelphia, Pa.
Philadelphia, Pa.: Camden, Gloucester, Burlington, N.J.
Essexr Middlesex, Norfolk, Plymouth, Suffolk, Mass.
Allegheny, Beaver, Washington, Westmoreland, Pa.
Albany, Schenectady, Rensselaer, Saratoga, N.Y.
Lackawanna, Pa.
Holyoke, Chicopee, Springfield, Mass.
Bristol, Newport, Providence, Kent, Washington, R.1.
Hartford, Tolland, Corm. Lake, Porter, Cook, Will, Kane, Ill.
Cook, DuPage, Kane, Lake, McHenry, II 1.
Macomb, Oakland, Wayne, Mich.
Milwaukee, Waukesha, Wis.
Hennepinr Ramsey, Anoka, Dakota, Washington. Minn.
Lake, Cuyahoga, Ohio Franklin, Ohio Buchanan, Mo.
Cass, N. Dak.: Clay, Minn.
Jefferson, St. Charles, St. Louis, Me.: Madison, St. Clair, Ill.
Bay, Mich.
Cass, Clay, Jackson, Platte, Mo.
Marion, I nd.
Montgomeryr Greene, Miami, Ohio Jackson, Mich.
Jefferson, Leavenwotihr Kans.: Platt, Mo.
Brown, Clinton, Ohio Rusk. Wis.
South St. Bernard, Jefferson, Orleans, La.
Washington, D. C.: Fairfaxr Arlington, Vs.: Prince Georges, Montgomery, Md.
Hancock, Hamblen, Hawkins, Claiborne, Term.
Barbour, Ala.
Clay, Calhoun, Roane, W. Va.
West Orange, Los Angeles, Calif.
Los Angeles, Cal if.
Alameda, Contra-Costa, San Mateo, San Francisco, Solano, Cal if.
ColIin, Denton, Dallas, Ellisr Tex.
Bexar, Tex.
Adams, Arapahoe, Denver, Jefferson, Boulder, Colo.
Sacramento, Calif.
Hunt, Rains, Tex.
Mason, Thurston, Wash.
Greeley, Nance, Nebr.
Camadian, Cleveland, Oklahoma, Okla.
‘ County, parish, or borough
in the examination. Whether or not an individual par-ticipates is determined by many factors, some of them uncontrollable, and, therefore, the outcome may be reasonably treated as a random event with a par-ticular probability of occurrence. If the probabilities of participation were known and were greater than zero for all persons, then the examined persons would constitute a probability sample from which unbiased estimates of the target population could be derived.
In this situation, the effect of nonparticipation would only be to reduce the sample size, thereby increasing the sampling errors of examination findings. In prac-tice, however, a potential for bias due to nonresponse exists because the exact probabilities are never known. A further potential for bias exists if: (1) a
sizable proportion of sample persons have a zero probability of participation, that is, they would never agree to participate in an examination survey of the same procedures and inducements, and (2) these per-sons differ from other sample persons with respect to characteristics under examination. It is for these reasons that intensive efforts were made in NHANES I to develop and implement procedures and induce-ments that would reduce the number of nonrespond-ents and thereby reduce the potential of bias due to nonresponse. These procedures and inducements are discussed elsewhere.4
NOTE: Alist of references follows the text.
Table 11. Population estimates for examination locations 1-65 by sex, race, and age at examination: United States, 1971-74
Estimated population
Age at examina tion Male Female
Total
All races White 1 Black All races White I Black
l-74years . . . . 193,976,381 94,239,866 82,740,899 10,413,986 99,736,515 86,867,546 11 ,99!9,935
6,672,346 3,378,503 2,866,374 485,872 3,293,&?3
6-7years . . . .
2,755,016 511,134
7,193,663 3,652,322 3,060,888 573,867 3,541,341
8-9years . . . .
2,951,927 5716,578
7,696,597 3,880,396 3,279,649 586,419 3,816,201
10-n years . . . .
3,257,936 53!3,855
8,465,793 4,381,730 3,732,593 563,823 4,064,063 3,424,070
12-14years . . . . 12,335,321
61’7,793
6,312,519 5,397,061 879,377 6,022,802
15-17years . . . .
5,122,189 8315,252
12,318,434 6,207,169 5,311,596 812,321 6,111,265 5,233,091 85:3,294
18-19years . . . . 7,352,200 3,673,321 3,206,467 404,045 20-24years . . . .
3,678,879 3,158,930 504,417
17,325,038 8,109,775 7,094,036 866,201 9,215,263
25-34years . . . .
7,972,486 1,07:3,358 26,936,001 13,002,514 11,594,115 1,231,793
35-44 years . . . .
13,933,487 12,160,578 1,646,337
22,268,477 10,675,731 9,515,530 1,004,953 11,592,746 10,111,458 1,3113,050
45-54years . . . . 23,313,316 11,150,110 10,039,124 1,056,837 12,163,206 10,879,167 55-64 years . . . .
1,23”7,459
19,049,001 9,072,586 8,274,948 702,647 9,976,415 9,037,157 87’1,098
165-74 years . . . . 12,773,574 5,496,351 4,969,903 486,257 7,277,223 6,603,303 65’1,579
llncl~desperaonsof mher races; see appendix Il.
NOTE: The numbars in this table constitute estimates and closely approximate the U.S. population as estimated by the U.S. Bureau of the Census as of November 1972.
SOURCE: First National Health and Nutrition Examination Survey nutrition examination sample, 1971-74.
Table Ill. Population estimates forexamination locations l-l OObysex, race, andage at examination: United Statesr 1971-75
Estimated population
Age at examination Male Female
Total
All races White I Black All races White I Black
Z25-74years . . . . 106,639,033 50,586,997 45,303,260 4,693,184 56,052,036 49,582,632 5,963,002
;25-34Vears . . . . 28,296,796 13,663,092 12,122,508 1,302,502 14,633,704 :35-44 years . . . .
12,712,842 1,736,498
22,302,278 10,761,322 9,578,852 1,023,520 ‘1T,540,956 10,003,331 1,391,510
45-54 years . . . . 23,548,824 11,288,375 10,131,455 1,095,092 12,260,449 10,921,890 1,26;!,609
!j5.64 years . . . . 19,345,852 9,191,996 8,336,468 768,237 10,153,856 9,164,012 90~,874 65-74 years . . . . 13,145,283 5,682,212 5,133,977 503,833 7,463,071 6,780,557 666,511
‘Ilnclude sper$onsof other races; see aPPendiX Il.
NOTE: The numbers in this table constitute estimates and closely approximate the U.S. population as estimated by the U.S. Bureau of the Celnsusas of February 1974.
SOURCE: First National Health and Nutrition Examinatiorr Survey detailed examination sample, 1971-75.
Despite these intensive efforts, 26 percent of the
sample persons from the 65-location nutrition exami-nation sample and 30 percent of the sample persons from the 100-location detailed examination sample were not examined. Consequently, the potential for a sizable bias does exist in the estimates in this publi-cation. From what is known about the nonrespond-ents and the nature of nonresponse, it is believed that the likelihood of sizable bias is small.
Efforts have been made using data from NHANES I and from an earlier sumey to examine possible health-related differences between examined and non-examined persons. Reasons for nonparticipation in NHANES I were investigated29 on a sample of 325
NOTE: A list of references follows the text.
persons (209 examined persons, 35 persons whet had made appointments for the examination but who never came to the mobile examination center for the examination, and 81 persons who refused to partici-pate in the survey). The sample persons for this study came from four stand locations: St. Louis, Monterey, New York, and Philadelphia. They were asked to indi-cate why they did not choose to be examined. The primary reasons given were that they had no need for a physical (48 percent), or that the examination times were inconvenient because of work schedules or c]ther demands (15 percent). Only 6 percent of those per-sons not examined in NHANES I indicated that they refused the examination because of sickness, and 3 percent based theix refusal on fears of possible findings.
Data on both examined and nonexamined (but Missing data interviewed) persons were analyzed30 by using
infor-mation from the first 35 stands of NHANES I. For the health characteristics compared, the two groups were quite similar. For example, 20 percent of the examined persons reported that a doctor had told them they had arthritis, compared with 17 percent of the unexamined persons. Similarly, 18 percent of both the examined and the nonexamined persons had been told by a doctor that they had high blood pres-sure. Twelve percent of both groups reported that they were on a special diet, and 6 percent of both groups said that they regularly used medication for nerves,
A study31 of factors relating to response in a health examination survey, based on data from Cycle I of the Health Examination Survey,30 showed that 36 percent of the unexamined people in that survey viewed themselves as being in exceIIent health, com-pared with 31 percent of the examined people. A self-appraisal of being in poor health was made by 5 percent of the nonexamined persons and by 6 per-cent of those who were examined. Additionally, a different study of Cycle I datas 2 showed that com-parisons between two extreme groups-those who participated in the survey with no persuasion and those who participated onIy after a great deal of per-suasion–indicated that differences between the two groups generally had little effect on estimates based on numerous selected examination and questionnaire items. This was interpreted as evidence that no large bias exists between the two groups for the items in-vestigated and was offered as further support for the belief that little bias is introduced to the findings be-cause of differences in health characteristics between examined and nonexamined persons.
As noted earlier, the data in this report are based on weighted observations, and one of the components of the weight assigned to an examined person was an adjustment for nonresponse. Because the probabilities of participation are not known for sample pelsons in NHANES I, a procedure was adopted that multiplies the reciprocal of the probability of selection of sam-ple persons by a factor that brings estimates based on examined persons up to a level that would have been achieved if all sample persons had been examined.
This nonresponse adjustment factor is the ratio of the sum of sampling weights for all sample persons within a relatively homogeneous class defined by age, sex, and income within each stand to the sum of sampling weights for all responding sample persons within the’
same homogeneous class from the same stand. If homogeneous groups can be defined that are also homogeneous with respect to the characteristics under study, the procedure can be effective in re-ducing the potential bias from nonresponse.
NOTE: A list of references follows the text.
Examination surveys are subject to the loss of information not orLIy through the failure to examine all sample persons, but also from the failure to obtain and record all items of information for examined per-sons (item nonresponse). In the case of Laboratory findings, missing data can result from such occur-rences as equipment failure, laboratory accidents, poor specimen preparation, and loss of specimens in the mail between the examination locations and testing laboratories.
Differential leukocyte count data are not available for 1,059 persons, or 15.3 percent of the 6,913 adults who received the detailed examination during the 100-location sample. White blood cell (WBC) counts for 2,072 sample persons could not be reported from the NHANES I nutrition examination sample (stands 1-65), and the counts for an additional 280 persons were discarded during quality control procedures dis-cussed in appendix IV.
After inspecting the age, sex, and race character-istics of that portion of the sample for whom WBC and/or differential leukocyte count data are missing, and after comparing these characteristics with those of the larger portion of the sample for which data are available, the assumption was made that the missing data accounts for little bias because of the nonsys-tematic nature of the reasons for missing laboratory data. Consequently, the weighted estimates that ap-pear in this report as means or percents reflect an imputation for missing data; that is, the findings are presented as if information had been gathered for all sample persons.
Small numbers
In tables 2-5 magnitudes are shown for cells for which the sample size is so small that the sampling error may be several times as great as the statistic itself. In such instances the numbers have been in-cluded to convey an overall impression.
Reliability of estimates
Since the statistics presented in this report are based on a sample, they will differ somewhat from the figures that would have been obtained if the sur-vey had been conducted on the complete population.
In other words, the statistics are subject to sampling variability y.
The standard en-or is primarily a measure of sam-pling variability, but it may also include part of the variation that arises in the measurement process. The standard errors presented in tables 1, 7, and 9 have been calculated by a technique referred to as balanced repeated replication. The need for this specialized technique for estimating standard errors arises because of the complexity of the sample design of NHANES I.
Estimates of standard errors are themselves subject to errors that may be large if the number of cases which the estimates are based is small.
Utilization of standard errors
Two examples can illustrate the use of the
Two examples can illustrate the use of the