Internal Validity

Authors
Affiliations

Nate Yomogida, B.S., SPT

Doctor of Physical Therapy

B.S. in Kinesiology

Chloë Kerstein, B.S., SPT

Doctor of Physical Therapy

B.A. in Neuroscience

Internal validity refers to the possibility that the the conclusions drawn from the results of the study accurately reflect the experiment itself1. Internal validity determines whether the study was setup in a way that will accurately answer the initial question without influence of other variables.

What is a study with internal validity?

“If a study is internally valid, this means that any effects/changes or lack thereof in the dependent variable can be directly attributed to the manipulation of the independent variable” -Polgar & Thomas1.

Types

  • Statistical Validity
  • Face Validity
  • Criterior-related Validity:
    • Concurrent Validity
    • Predictive validity
    • Prescriptive validity
  • Content validity
  • Construct validity
  • Convergent
  • Discriminant (Divergent)

Threats to Internal Validity

Systematic Error

  • Attrition Effects

  • Maturation Bias refers to the error of assuming group change over time is due to the independent variable and not natural improvement over time.

  • Selection bias / selection effects

  • Testing Effects: Systematic error created by a problem with the experimental setup that will create a difference in one group and not the other.

  • Ambiguous Causation: Error that occurs when researchers incorrectly interpret correlations/associations between variables as causal effects.

  • History Effects

  • Regression of the Mean

References

1.
Sim J, Arnell P. Measurement validity in physical therapy research. Physical Therapy. 1993;73(2):102-110; discussion 110-115. doi:10.1093/ptj/73.2.102

Citation

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