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I
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Randomized controlled trials of adequate spatial and temporal scale for the study species
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II
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Controlled trials without randomization with adequate spatial and temporal scale for the study species
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III
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Comparisons of differences between sites with and without controls with adequate spatial and temporal scale for the study species.
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IV
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Evidence obtained from multiple time series or from dramatic results in uncontrolled experiments.
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V
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Opinions of respected authorities based on qualitative field evidence, descriptive studies or reports of expert committees.
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VI
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Evidence inadequate owing to problems of methodology e.g. sample size, spatial or temporal scale.
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