Level 1:	Strong evidence from at least 1 systematic review of multiple, well-designed RCTs.
Level II:	Strong evidence from at least 1 properly designed RCT of appropriate size.
Level III:	Evidence from well-designed trials without randomization, single-group pre-post, cohort, time series, or matched-case controlled studies.
Level IV:	Evidence from well-designed nonexperimental studies from more than 1 centre or research group.
Level V:	Opinions of respected authorities, based on clinical evidence, descriptive studies, or reports of expert committee.