Table 5.

Level of evidence according to the 2005 classification system of the Dutch Institute for Healthcare Improvement CBO.

	Intervention
A1	Systematic review of at least two conducted independent of each other were of evidence level A2
A2	Randomized double-blinded comparative clinical research of good quality and efficient size
B	Comparative research but not with all characteristics as mentioned for A2. This includes also patient-control research and cohort research
C	Not comparative research
D	Opinion of experts