Observational longitudinal research is particularly useful for assessing etiology and prognosis and for providing evidence for clinical decision making. However, there are no structured reporting requirements for studies of this design to assist authors, editors, and readers. The authors developed and tested a checklist of criteria related to threats to the internal and external validity of observational longitudinal studies. The checklist criteria concerned recruitment, data collection, biases, and data analysis and descriptive issues relevant to study rationale, study population, and generalizability. Two raters independently assessed 49 randomly selected articles describing stroke research published from 1999 to 2003 in six journals: American Journal of Epidemiology, Journal of Epidemiology and Community Health, Stroke, Annals of Neurology, Archives of Physical Medicine and Rehabilitation, and American Journal of Physical Medicine and Rehabilitation. On average, 17 of the 33 checklist criteria were reported. Criteria describing the study design were better reported than those related to internal validity. No relation was found between study type (etiologic or prognostic) or word count and quality of reporting. A flow diagram for summarizing participant flow through a study was developed. Editors and authors should consider using a checklist and flow diagram when reporting on observational longitudinal research.

While this population is a difficult one to reach for research, attempts to do so should not be abandoned, because the potential health benefits for this underserved group far outweigh the recruitment barriers experienced.

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Narrow inclusion criteria may offer benefits such as increased precision and reduced loss to follow-up, but there are important disadvantages, such as uncertainty about extrapolation of results, which may result in denial of effective treatment to groups who might benefit, and delay in obtaining definitive results because of reduced recruitment rate. Selective participation by teaching centres and sicker patients in treatment RCTs may exaggerate the measured treatment effect. Prevention trials, on the other hand, may underestimate effects as participants have less capacity to benefit.

What do we do if different studies appear to give different answers? When applying research to questions for individual patients or for health policy, one of the challenges is interpreting such apparently conflicting research. A systematic review is a method to systematically identify relevant research, appraise its quality, and synthesize the results. The last two decades have seen increasing interest and developments in methods for doing high quality systematic reviews. Part I of this book provides a clear introduction to the concepts of reviewing, and lucidly describes the difficulties and traps to avoid. A unique feature of the book is its description, in Part II, of the different methods needed for different types of health care questions: frequency of disease, prognosis, diagnosis, risk, and management. As well as illustrative examples, there are exercises for each of the sections. This is essential reading for those interested in synthesizing health care research.