The use of large data bases in health care studies.

The growing number of large health data bases available represents a valuable resource for health care research. Many available data bases, however, have subtle and/or complex defects in their design as well as in the quality of the data themselves. The apparent ease and economy of using pre-collected data cannot eliminate the need for careful selection, examination, and analysis of these data. Existing documentation should be critically reviewed to assess the appropriateness of the data base for its intended use. Once in hand, the completeness and coding of the data should be examined in detail before attempting to test hypotheses. In conducting data analysis, the investigator must be aware of the potential problems related to the size of the data base, the unit of analysis, and the sampling strategy--particularly if sampling involved stratification or clustering. Awareness of the potential pitfalls inherent in the use of large health data bases can help prevent many problems and disappointments, as well as improve the validity and efficiency of statistical analysis.