What is the most important validity to interrogate in causal claims?

The answer is construct validity. This is used to associate and interrogation of causal claims. Through determining the variable, interrogation is already being done to see if the variables have an effect on what is being measured.  This way construct validity is being done. Also, one variable can be studied in different angles to understand the degree of how it takes effect or how can it be associated.

Source

http://nortonbooks.typepad.com/files/research-methods-in-psychology-sample-chapter-3-1.pdf