Evaluation of research studies. Part II: Observational studies

Clinical practice is often based on the results of research. Critical evaluation of research studies is important if appropriate conclusions are to be drawn. In this series of columns, we review principles of research methodology and statistical analysis. Our intent is to assist certified nurse-midwives in understanding the relative merits of the research they read and use. This paper, the second of the series, will review issues pertinent to observational study design.