Monitoring Human Tissues for Toxic Substances (1991) / Chapter Skim
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4. Sampling Methods
4. Sampling Methods
Pages 71-98
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From page 71... ...
When human populations are studied, many of those steps can be carried out only imperfectly. Problems in developing a true probability sample are common, serious measurement errors might be unavoidable, and compromises in the definition of the target population are sometimes necessary.
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From page 72... ...
It deals only with sampling; measurement methods are discussed elsewhere. We describe ideal sampling practices, the compromises that are often necessary because some parts of the population are inaccessible or because of budgetary restrictions, the sampling methods used in the NHATS, and implications for drawing conclusions about the U.S.
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From page 73... ...
The sample design in a national survey is thus usually fairly complex, and the statisticians responsible for the sample-selection methods normally go through a series of steps: · Consider the reasons for and effects of foreseeable failures in various possible sampling schemes, such as incomplete information on some sampled persons. · Determine what the sampling stages should be; cost, logistics, and information that is or could be made available for sample selection at each stage should be considered.
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From page 74... ...
Statisticians generally consider that the development of a sample design should include procedures for estimating sampling errors. Most national surveys of human populations use complex sample designs, with multistage sample selection, stratification, and probabilities of selection that differ among subpopulations.
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From page 75... ...
Some common examples follow. · Omission of institutional populations from the sampling frame, even though one would prefer to consider them as part of the target population.
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From page 76... ...
When refusal to cooperate is especially common among a particular segment of the population that differs in important ways from the population at large, even a small nonresponse rate can produce serious biases in the statistics. Responsible survey organizations put considerable effort into attaining as high a response rate as feasible.
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From page 77... ...
THE NATIONAL HUMAN ADIPOSE TISSUE SURVEY SAMPLE Goals of Study and Target Population NHATS goals are expressed in very general terms. The committee could find no clear articulation either of its primary objectives and priorities or of the precision needed in detecting magnitudes of pollution or year-to-year changes.
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From page 78... ...
population, but the subjects on whom measurements are made are an uncontrolled combination of recently deceased persons and surgical patients. There is no attempt to keep the mix of deceased and surgical patients constant from year to year.
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From page 79... ...
There seem to be no strong efforts to persuade reluctant examiners to change their minds: no contacts are made by high-level EPA officials, state officials, or local medical society representatives emphasizing the importance of the study; and the Midwest Research Institute (MRI) operations manual on procedures to be followed in data collection does not provide any motivation for a contractor's field representatives to attempt to convert refusals.
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From page 80... ...
Similarly, the letters in the recruitment package use the phrases "invitation to participate" and "we hope that you mill assist." The attitude seems to be that it is not important who provides the specimens. According to MRI, within each SMSA one or more hospitals and associated pathologists or medical examiners are selected and asked to supply adipose tissue specimens.
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From page 81... ...
With probability sampling, the selection of first-stage units with probability proportionate to size and the use of properly calculated fixed quotas of persons within the first-stage units provide close to equal probability samples for each of the designated sex-age-race categories. The quotas have to be revised periodically to reflect changes in the distribution of the population among the first-stage units.
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From page 82... ...
This section discusses improvements that are possible, at a modest cost, within the present framework of the NHATS. Although the committee recommends that adipose tissue be replaced by blood as the primary human tissue monitoring program, it should be retained, with improvements, during a transition, and possibly longer.
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From page 83... ...
public on the existence of contaminants in human tissue. The questions need to be expressed in more quantitative terms than the general goals that have been stated in the past (and stated elsewhere here)
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From page 84... ...
p can be estimated from past data or other previous studies. The design effects will not be known in advance of introducing a probability sample.
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From page 85... ...
The 95% chance implies that two standard errors is equal to 0.02, or ~ = 0.01. The formula n = dpq/~2 with d = 2, results in a sample size of 1800.
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From page 86... ...
That implies that, if one wishes to achieve a particular level of precision, the necessary sample size is independent of whether the population is that of the total United States, a region, or a subdomain defined in another way, such as males, rural residents, or persons at least 45 years old. As a result, the total sample size depends on the number of subdomains for which prespecified precision is needed (as well as the standard error desired)
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From page 87... ...
There is no way to know whether cadavers or patients in cooperating institutions are similar to those in noncooperating institutions. Most responsible survey organizations put considerable effort into attaining as high a response rate as feasible, and EPA, in its operation of the NHATS should have the same goal.
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From page 88... ...
The MRI Operations Manual instructs field personnel to make calls to remind cooperating pathologists to check on progress, but does not emphasize the importance of meeting the quotas. Perhaps the field staff itself does not recognize that this is a major requirement or is too diffident in its dealings with medical examiners and pathologists.
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From page 89... ...
Some light might be shed on the subject by examining differences in contamination concentrations among hospitals in the same SMSA, or by comparing data on SMSAs before and after some hospitals were changed. Composite Specimens Starting with tissues collected in 1982, the EPA stopped analyzing adipose tissue specimens from separate individuals and started analyzing"composites." Individual specimens for immediate analysis (but not archived specimens)
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From page 90... ...
Perhaps it could be done by examining the data for the years in which individual specimens were analyzed separately. Statistical modeling is currently used to estimate mean concentrations by geographic regions, three age groups, sex, and race.
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From page 91... ...
. The computation of standard errors in most statistical software packages is not appropriate for a multistage sample design that uses variable weights.
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From page 92... ...
The Benefits of Blood Collection for Probability Sampling From a statistical point of view, a troublesome aspect of the entire NHATS program has been replacing the U.S. population as the target population with surgical patients and cadavers subject to autopsy.
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From page 93... ...
Blood specimens could be collected, in accord with procedures roughly similar to those in NHANES, from subjects that are close to a true probability sample of the U.S. population.
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From page 94... ...
Despite the value of those additional features of a program based on blood, it must be clearly understood that the primary rationale is the need for true probability sampling, and that the advantages of having a probability sample must be protected in other aspects of study design. For example, EPA's plans for a National Blood Network (NBN)
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From page 95... ...
Although the broad scan analysis provides a substantial amount of information not available with the original measurement method, it does not satisfy all the purposes of a human tissue monitoring program. In deciding to use broad scan analysis, EPA acted as though it had a fixed budget for monitoring and as though the higher costs per chemical analysis had to be compensated for with a smaller number of samples analyzed.
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From page 96... ...
The basic problem is that a broad sample would include only a handful of persons in any small group of special concern, and design of a sample to answer questions about such groups would either require an enormous equalprobability sample or distort a weighted sample to the point where it might be unsuitable for its basic purpose, even with appropriate analyses to adjust for differential weights. An agency staff qualified to maintain a strong human tissue monitoring program should have skills and facilities for investigation of risks in special populations, and special studies might well be assigned to them.
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From page 97... ...
Although most population surveys find it necessary to compromise somewhat on ideal standards, the departures from probability sampling in the NHATS are far in excess of what most statisticians would consider acceptable. The main deficiencies are these: Although the target population is the living U.S.
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From page 98... ...
The sampling plan can be palmed after He one used u' NAMES, whose subjects an' close to a true pro;b~ibili~ sample of the U.S. pop~r~ In addition to having a probability sample, the system would permit interviews to be conducted with the sampled persons to obtain data on covariates.
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