A study or report that appears in the published literature can
be included as evidence in the development of an advisory if it
meets four essential criteria. Failure to meet one or more of these
criteria means that a study had features that did not make it
suitable for analytic purposes. The four essential criteria are as
follows: (1) the study must be related to one of the specified
linkage statements; (2) the study must report a clinical finding or
set of findings that can be tallied or quantified. This criterion eliminates reports that contain only opinion;