Explain and identify threats to internal and external validity in research methods?

Answer 1

here's the links you need:

internal: http://en.wikipedia.org/wiki/Internal_validity

external: http://en.wikipedia.org/wiki/External_Validity

good luck

Answer 2

"One major source of confounding arises from non-random patterns in the membership of participants in the study, or within groups in the study. This can affect internal and external validity in a variety of ways, none of which are necessarily predictable. It is often only after doing a great deal of work that we discover that some glitch in our procedures or some oversight has rendered our results uninterpretable."