3: Evidence obtained from well designed non-experimental studies, such as comparative studies, correlation studies, and case reports.
This level includes all non experimental studies- cross-sectional, comparative, correlation, etc. It also includes case reports and case series.
However, the emphasis is on “well designed” studies. Not all studies satisfy this basic requirement. Therefore, careful scrutiny of the methodology is crucial.
4: Evidence obtained from expert committee reports, or opinions or clinical experience of respected authorities.
This is the lowest level of evidence. Ironically, this forms the basis of a majority of healthcare practices. The reasons are not hard to find, and have been described in the post titled ‘Evidence: The problem of plenty’.
It is sobering to note that expert opinion (by renowned seniors) is not valued as much as a case report (probably written by a junior professional). This one fact should prompt us to discard ‘time-honoured’ practices if they are not supported by at least level 3 evidence.