includes 471 hypertension guidelines and 276 stroke guidelines (NGC, 2007e,f). Anyone looking to ferret out pertinent information faces a substantial sifting process and challenges in determining which of the guidelines are the most relevant and trustworthy. Although the NGC allows readers to compare the guidelines side by side across a number of dimensions, this feature quickly becomes unwieldy as the number of relevant guidelines increases. In addition, the guidelines differ substantially in the way that they present information, making it difficult for the reader to compare one set of findings directly against another. For example, guidelines employ different rating scales to characterize the quality of the supporting evidence (see below).

Despite the overabundance of clinical guidance in some topic areas, little guidance exists in other important areas. The following examples illustrate how gaps in guideline production may occur:

• Some commonly used treatments may not have been examined in systematic reviews, primarily because of a lack of agreement on which professional society “owns” the condition (e.g., treatments for prostate cancer, which may be “owned” by the American Urological Association, the American Society of Clinical Oncology, or the American Society for Therapeutic Radiology and Oncology).

• Researchers may avoid doing reviews of treatments for rare and “orphan” diseases either because the evidence is weak, because no entity is identified as being responsible for developing a guideline, or because there is inadequate financial support to conduct the work.

• Some professional societies may not produce guidelines at all because they do not view it as a part of their mission, or they may release clinical position statements that have very little evidentiary basis.

• Given the speed at which medicine is changing, guideline production by professional societies may fall behind what is known about new knowledge and technology.

Recognizing that in some clinical areas multiple organizations may seek to develop guidelines, some groups have developed collaborative activities that promote consensus in clinical practice guidelines. For example, the