When looking into levels of evidence you will find that a heirarchy exists.  This heirarchy categorizes the types of studies and evidence that you may find when engaging in research.  The highest level of evidence is that of a randomized controlled trial with n=1. 

Randomized Controlled Trial (RCT) with n=1   
Systematic Review of RCT's
Multiple RCT's
Randomized Controlled Trial (RCT)
Systematic Review of the observational cohort or case control studies or large case series
Observational Cohort or Case Control Studies or a Large Case Series
Case Reports, Small Case Series
Unsystemtic Clinical Observations

The following systems for defining levels of evidence are based on study design.   http://en.wikipedia.org/wiki/Evidence-based_medicine