The following are links to sites discussing the concept of validity in studies:
http://www.socialresearchmethods.net/kb/intval.php
http://vault.hanover.edu/~altermattw/methods/assets/Readings/Internal_Validity.pdf
http://en.wikipedia.org/wiki/Validity_(statistics)
http://en.wikipedia.org/wiki/Internal_validity
http://en.wikipedia.org/wiki/External_validity
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2920077/pdf/jpm.2008.9690.pdf
http://www.natco1.org/research/files/Validity-ReliabilityResearchArticle_000.pdf
http://www.uniteforsight.org/research-methodology/module3
http://www.nova.edu/ssss/QR/QR8-4/golafshani.pdf
http://www.une.edu.au/WebStat/unit_materials/c2_research_design/validity_threat_internal.htm