Thank you very much for the references; they are as complete as it gets. I was looking for the possible impact of internal & external validity in relation to data integrity, correlation and causality.
For internal validity, how well you implement your methodology i.e whether it has measured what it your research question has intended to or not, so threats are poorly designed research tools and techniques, not qualified field researchers, male asking sensitive questions to female participants (few examples). For external validity is about your sampling procedure, whether or not your sampling design has included the heterogeneous characteristics of your study population so that your findings are representative of them.
Data collection can be affected by what is considered ethical in the area of study. (Babbie, 2017). In which case the data can be biased, despite the use of quantitative methods. This in itself affects both the internal and external validity of the study, because the manipulation of the variables can easily lead to false conclusion.
Reference
Babbie, E. (2017). Basics of social research (7th ed.). Boston, MA: Cengage Learning.
Threats to internal validity in quantitative research include poor design of research tools and techniques, using unqualified field researchers, and asking sensitive questions to individuals of the opposite gender.