Table 2 Definition of level of evidence

From: Summary version of the standards, options and recommendations for nonmetastatic breast cancer (updated January 2001)

Level A

There exists a high standard meta-analysis or several high-standard randomised clinical trials which give consistent results

Level B

There exists good quality evidence from randomised trials (B1) or prospective or retrospective studies (B2). The results are consistent when considered together

Level C

The methodology of the available studies is weak or their results are not consistent when considered together

Level D

Either the scientific data does not exist or there is only a series of cases

Expert agreement

The data does not exist for the method concerned but the experts are unanimous in their judgement

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