What are the 12 threats to internal validity?

The 12 main threats to internal validity are:

  1. History: Changes in the environment or events that occur outside of the study can affect the outcome.
  2. Maturation: Changes in the participants over time (e.g., age, skill level) can affect the outcome.
  3. Testing: The act of testing or measurement itself can affect the outcome (testing effect, practice effect, or carryover effect).
  4. Instrumentation: Changes in the measuring instrument or tool used to collect data can affect the outcome.
  5. Statistical regression to the mean: The tendency of extreme scores to regress towards the mean, which can lead to a loss of statistical significance.
  6. Selection: The selection of participants for the study can affect the outcome (selection bias), especially in the case of non-probability sampling.
  7. Experimental mortality or attrition bias: The loss of participants or dropouts during the study can affect the outcome.
  8. Multiple-treatment interference: The interaction between different treatments or conditions can affect the outcome.
  9. Social desirability bias: The participants’ awareness of being in a study and their desire to be well-liked by researchers can affect the outcome.
  10. Social interaction: The participants’ awareness of being treated differently than people in other groups can affect the outcome.
  11. Residual confounding: The presence of unmeasured or uncontrolled extraneous or confounding variables that affect the outcome and are not accounted for in the analysis.
  12. Order effect: The order of the independent variable levels affects the dependent variable.

There are several ways to counter these threats to internal validity, for example, through randomization, the addition of control groups, and blinding.