The studies may be interventional, may be observational, or may involve provider or patient self-reports or record reviews.
In some cases, the best available evidence may reflect clinical/professional experience or expert consensus opinion. All sources may yield information relevant to patients and clinicians, who may be required to make management decisions before the best possible evidence is available.
In view of this complexity, the PDQ Supportive and Palliative Care Editorial Board took a pragmatic approach to defining the levels of evidence that it would utilize. The levels used by the Board focus primarily on the strength of a study's design as appropriate to the questions being investigated. These levels should be easily understood by health professionals and serve as a starting point for discussing the strength of study results.