Lack of Internal Validity - A Complex Example

Positive Affect and the Complex Dynamics of Human Flourishing by Barbara L. Fredrickson and Marcial F. Losada is a popular article published in 2005.
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3126111/


But see this critique on the above paper

The Complex Dynamics of Wishful Thinking: The Critical Positivity Ratio.
Brown, Nicholas J. L.; Sokal, Alan D.; Friedman, Harris L.
American Psychologist, Jul 15 , 2013, No Pagination Specified. doi: 10.1037/a0032850

We examine critically the claims made by Fredrickson and Losada (2005) concerning the construct known as the “positivity ratio.” We find no theoretical or empirical justification for the use of differential equations drawn from fluid dynamics, a subfield of physics, to describe changes in human emotions over time; furthermore, we demonstrate that the purported application of these equations contains numerous fundamental conceptual and mathematical errors. The lack of relevance of these equations and their incorrect application lead us to conclude that Fredrickson and Losada's claim to have demonstrated the existence of a critical minimum positivity ratio of 2.9013 is entirely unfounded. More generally, we urge future researchers to exercise caution in the use of advanced mathematical tools, such as nonlinear dynamics, and in particular to verify that the elementary conditions for their valid application have been met.
http://psycnet.apa.org/psycinfo/2013-24609-001/


According to the criticism, the research paper by Frederickson and Losada does not have internal validity.