In this age of evidence-based medicine there is a scale to judge how "good" the evidence is:
Ia: metaanalysis of RCT
IIa: controlled trial
IIb: experimental trial
III: case series, case-control studies
IV: 'expert' opinions
Remember these because the odd MRCP question asks you what level of evidence a particular study is providing.
To the final MRCP revision battle of the day...