Quality of Research Evidence: Levels

A grading system was developed by which to judge the quality of research on medical treatments as follows:

• Level A - High A finding that additional research is unlikely to change the estimate of the effect of the treatment. This means that there are several good quality studies that agree in their findings, or a very large, high quality study.
• Level B - Moderate A finding that further research may have an impact on the estimate of the effect of the treatment. This means there is only one high quality study, or there are several research studies that have limitations, such as the sample size, placebo use, double-blind or not - etc.
• Level C - Low A finding that further research is likely to change the estimate of the effectiveness of the treatment. This means there is only one study with limitations, such as a single pilot study with a small sample size. Additional research is needed.
• Level D - Very low A finding that any estimate is uncertain. This means there is no direct evidence or study with sufficient significance or only an expert opinion. Proper research is needed.