Table 2 Generic hierarchy of quality of evidence based on the type of research undertaken for environmental evidence

I

Strong evidence obtained from at least one properly designed; randomised controlled trial of appropriate size.

II- 1

Evidence from well-designed controlled trials without randomisation.

II- 2

Evidence from a comparison of differences between sites with and without (controls) a desired species or community.

II- 3

Evidence obtained from multiple time series or from dramatic results in uncontrolled experiments.

III

Opinions of respected authorities based on qualitative field evidence, descriptive studies or reports of expert committees.

IV

Evidence inadequate owing to problems of methodology e.g. sample size, length or comprehensiveness of monitoring or, conflicts of evidence.