NHAMCS Data Collection and Processing

The U.S. Bureau of the Census acts as the field data collection agent for the NHAMCS. The actual visit sampling and data collection for the NHAMCS is primarily the responsibility of hospital staff. They are instructed in how to complete each survey item by field representatives. Different encounter forms were developed for use in the ED and OPD. The top section of each form, which contains the patient’s name and record number, is separated from the bottom section by a perforation running across the page. The top section remains attached to the bottom until the entire Patient Record form is completed. To ensure confidentiality, before collecting the completed Patient Record forms, the top section is detached and given to hospital staff. They are instructed to keep this information for 4 weeks, in case it is necessary to retrieve missing information or clarify ambiguous entries.

In addition to the completeness checks made by field staff, clerical edits are performed upon receipt of the data for central processing. Detailed editing instructions are provided to manually review the forms and to reclassify or recode ambiguous entries. Computer edits for code ranges and inconsistencies are also performed.

All medical and drug coding and keying operations are performed centrally by Constella Group, Inc. and subject to quality control procedures. The keying error rate for nonmedical items is typically less than 1 percent. For items requiring medical coding, discrepancy rates are generally under 2 percent.

Item nonresponse rates are generally 5 percent or less for NHAMCS data items, with some notable exceptions. These are described in detail in the complete NHAMCS downloadable documentation. Imputations for missing OPD data are performed for patient year of birth, sex, and race. Beginning in 2003, three additional items -- patient ethnicity, has patient been seen in this clinic before, and if so, how many past visits in the last 12 months -- were imputed. Ethnicity and seen before were last imputed in 1996; past visits had not been imputed previously. On the 2003 ED file, patient year of birth, sex, race, ethnicity, and immediacy with which patient should be seen were all imputed. Patient ethnicity had not been imputed since 1996.

Drug data are coded using a unique classification scheme developed at NCHS. Listings of drugs by entry name (the name used by the respondent to record the drug on the Patient Record form) and by generic substance are available. The therapeutic class of drugs is based on the National Drug Code Directory.

For users who wish to review a list of patient and drug data items available over multiple years of the survey, refer to the Electronic Data Products web pages.