"Internal validity is the extent to which a research study establishes a trustworthy cause-and-effect relationship." - San Jose

"Internal validity is not a "yes or no" concept. Instead, we consider how confident we can be with study findings based on whether the research avoids traps that may make those findings questionable. The less chance there is for "confounding," the higher the internal validity and the more confident we can be" - Kemper CJ.

"External validity examines whether the findings of a study can be generalized to other contexts." - Egger, Matthias et.al.