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National Information Center on Health Services Research and Health Care Technology (NICHSR)

Population controls
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Another approach is to rely on external population controls—such as those derived from a census—to adjust estimates.

Population or household surveys rely on demographic controls to achieve representativeness that cannot be achieved by full implementation of their protocols.

Unstated Assumption
When a survey cannot achieve full response from those selected for interview, statisticians may weight to reflect the demographic composition found in the census. This assumes that conditions are more homogeneous within demographic groups than between them.

Reporting entities and intermediaries
Some surveys rely on intermediaries to collect data.
For example, some surveys of health care services rely on data collected by hospital staff as the basis for their statistical records.
Yet, staffs have their own perspectives, goals and priorities which may shape the information reported.

Questionnaires and survey instruments
These tools rely on standardized data collection which might not achieve the highest level of individual accuracy. One problem is that analysts might not be able to relate these results as well to individual physician’s diagnoses.
 
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