Threads to Internal and External Validity

Threads to Internal and External Validity

1. Internal

   • Maturation: changes over time, caused by biological or psychological forces.
   • Selection: systematic bias not random to choose subjects.
   • Testing: a pretest is part of the experiment after a posttest testing can be a threat to this validity.
   • Instrumentalization: those conditions within a testing situation, other that the abilities of the subject, which may affect performance.
   • Regression: A class that scores particularly low can be expected to score slightly higher just by chance. Likewise, a class that scores particularly high, will have a tendency to score slightly lower by chance. The change in these scores may have nothing to do with the treatment.
   • Mortality: All of the high or low scoring subjects may have dropped out or were missing from one of the groups. Loss of subjects.
   • History: Something may happen at one site during our study that influences the results.

2. External

   • Multiple Treatment Interference: If a researcher applies several treatments, it's difficult to determine how well each of the treatments would work individually. It might be that only the combination of the treatments is effective.
   • Reactive Arrangements: Hawthorne effect, that shows how research must consider what participants know about a research experiment. Subjects may perform differently because they know they are being studied.
   • Experimenter Effects: A thread were by the presence of an experimenter can change the effectiveness of the treatment. Given a different person, the treatment might not work at all.
   • Pretest Sensitization: treatment might only work if a pretest is given. because the subjects may be more sensitive to the treatment.