Threats to Internal Validity
- Placebo Effect
- Subjects beliefs about the potential outcome or beliefs about what should occur can greatly influence their behavior. This can also occur when a subject is knowingly in the control group and either tries harder or gives up because of this.
- Experimenter Bias
- Experimenters, those administering the treatment, can bias the results knowingly and unknowingly depending on their beliefs about the results and their stake in the results.