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internal validity and threats to internal validity

Internal Validity

The extent to which the intervention rather than extraneous factors account for the results of the experiment

Threats to Internal Validity

1) History
2) Maturation
3) Testing (Repeated Testing)
4) Instrumentation
5) Statistical Regression
6) Selection Biases
7) Attrition
8) Reaction of Controls & Special Treatment
9) Diffusion of Treatment & Imitation of Treatment
10) Combination of Selection and Other Threats


Events occurring within or outside experiment that may account for the results


Internal changes within the participant that occurs over time and may account for the results

(Repeated) Testing

Practice or familiarity with a test may influence performance at the second testing not the intervention


Changes in the instruments or procedures over time may account for the results

Statistical Regression

The tendency for extreme scores to move toward the mean after repeated testing may account for the results

Selection Biases

Systematic differences in the group due to the way they were selected or assigned to groups may account for the results


Loss of participants in systematic direction may account for the results

Combination of Selection and Other Threats

threats of internal validity vary across groups making the threat selective for one group and not the other
Selection + Another Threat to Internal Validity

Diffusion of Treatment

The treatment accidentally is given to some or all control group participants

Special Treatment

Control group is not given the treatment but is given special attention

Reaction of Control

Control group realizes that it is not receiving the treatment and act in ways that obscures the results

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