According to Vogt (2007), some of the most common threats to validity in quantitative research are self-selection effects, volunteer effects, attrition, history effects, maturation effects, and communication among subjects.
This occurs if subjects are not randomly assigned to the groups that interest the researcher (Vogt, 2007). This can mean that the subjects assign themselves or choose their own groups; not representative of random assignment. Researchers should be sure to assign the groups to avoid self-selection.
This occurs because people cannot usually be studied without their prior consent, but those who give their consent are likely to differ in important ways from people who do not consent (Vogt, 2007). When possible researchers can study people without their consent by observing their public behavior and using the information when applicable.
Attrition occurs when subjects drop out or choose to no longer participate in a study. Researchers can help avoid high levels of attrition by careful screening of subjects.
History effects refers to dangers to validity that occur as a result of extended periods of time that pass during a study. According to Vogt (2007), one way to guard against the potential threats to the validity of history effects is to take frequent measurements of the outcome variable rather than just one measurement at the end of the study.
Maturation effects also occur as a result of extended periods of time that pass during a study. However, in this case the change that occurs is due to the individual development of participants in the research, not to external events (Vogt, 2007). Here again recurrent measurements can help guard against the risks of subject maturation.
Communication Among Subjects
Communication among subjects and the complications it can cause, can take many shapes. To help mitigate this Vogt (2007), recommends assigning institutions rather than individuals, to control and treatment groups.
Vogt, P.W., (2007). Quantitative Research Methods for Professionals. Boston, MA: Allyn and Bacon.