Psyc 235 internal and external validity
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PSYC 235: Internal and External Validity. Internal Validity. Definition “truth” of causal conclusions Established by the absence of confounding Internal “soundness” of experimental design. Threats. History Problematic for repeated measures (within groups) designs

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Internal validity
Internal Validity

  • Definition

    • “truth” of causal conclusions

    • Established by the absence of confounding

    • Internal “soundness” of experimental design


Threats
Threats

  • History

    • Problematic for repeated measures (within groups) designs

    • IV1 (test) SHIT HAPPENS IV2 (test)

      • IV the cause?

  • Maturation

    • Problematic for repeated measures

    • Time-related change to individuals


  • Testing

    • Problematic for repeated measures designs

    • Practice effects, reactive measures

  • Instrumentation change

    • Shifting measurement criteria

  • Regression to the mean

    • Interaction with selection

  • Selection bias

    • Problematic for between-subjects (independent groups designs)


  • Mortality

  • Interactions between other confounds and selection


External validity
External Validity

  • Definition

    • Generalizibility

    • To what extent can results be applied to other people, situations and contexts?

  • Types of generalization

    • Population

    • Environment – other places, situations

    • Temporal – other times


Methodological threats
Methodological Threats

  • Interaction of testing and treatment

    • Treatment sensitizes subjects for further manipulations

  • Interaction of selection and treatment

    • Only selected types of participants will respond to treatment

  • Reactive arrangements

    • Experiment produces “artificial” behavior

    • Demand characteristics

  • Multiple-treatment interference

    • Sequence effects


Participant threats
Participant Threats

  • On write rats and college sophomores….


Ensuring external validity
Ensuring External Validity

  • Replications

    • Direct replication

    • Systematic replication (replication with extension)

    • Conceptual replication (replication with extension)


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