Type of Evidence

What is it?

How to Evaluate the Quality




When we rely on "common sense" or "gut feeling" or "hunch"


Sometime "intuition" really comes from a vast well of past experience and knowledge


Intuition is private, there is no way to judge its reliability or dependability




Expert sources -- relying on others to interpret the evidence


Primary sources (direct observation)  is stronger while Secondary sources (hearsay) are weaker in quality.


Example: A research article from Journal of the American Medical Association (primary source) is a better source than a discussion of that same article in a popular news magazine such as Newsweek (Secondary source)


Personal experience & observations


The direct participation or observation an activity or event


Primary sources (direct experience or observation)  is stronger


Be wary of:

Selectivity personal experiences vary greatly

Personal interest something to gain from believing their testimony

Omitted information rarely provide sufficient info about the basis for their judgment


Case studies & examples


The systematic observation of a single individual or event


Look to see how systematic and encompassing the observation are. How have the observation accounted for bias of perspective and experience?


Be wary of vivid case studies. They should be viewed more as striking examples than as proof. Vivid examples demonstrate the outlying possibilities rather than the norm.




Uses similarity as a form of evidence -- If two things are alike in one aspect, then they will probably be alike in other respects as well.


Focus on 2 factors:

  1. The number of ways the two things are similar or different
  2. The relevance of the similarities and the differences


Be wary of catchy or folksy slogans that sound persuasive, but may have little relevance to the topic under discussion


Research studies


Systematic collection of observations to avoid bias

Publically verifiable data data obtained under conditions that other qualified people can make similar observations and obtain similar results

Control minimizing extraneous factors that might affect the accuracy and interpretation of generalization

Precision in language tries to be concise and consistent in use of language


What is the quality of the source?  Most dependable research studies are presented in peer-reviewed journal articles


Are there special strengths in the research methodology?


Has the study been replicated?


How far can we generalize the research studies



  • Research results that contradict the vast majority of other studies. 
  • Sometimes this can reflect dramatic new knowledge and a paradigm shift, but it can also mean poor quality or biased science



Science neither proves nor disproves. It accepts or rejects ideas based on supporting and refuting evidence, but may revise those conclusions if warranted by new evidence or perspectives. The concept of absolute proof is not scientific