The key to experimental design

At the end of the DVs it is good to have a manipulation check
    - These often give away the purpose of the study, so do them last!
 

Experimental Realism

Does the manipulation have an impact on the participants?

Are participants involved in the research paradigm/situation?
 

Internal Validity

Confidence that the manipulation of the IV is responsible for the changes in the DV(s)

Experimental control and standardization of conditions lead to high internal validity

There are a variety of threats to internal validity...
 

Confounds

Unmeasured variables that vary systematically across conditions

Avoid by: random assignment to conditions whenever possible
    - Never run an entire condition at once!!!
 

Placebo Effects

Avoid by: maintaining standardized expectations among P's across conditions

Examples:
    - Alcohol and attractiveness
    - Herbal pills and memory
    - Experimental medication and clinical trials
 

Demand Characteristics

When participants can guess the research hypothesis

Avoid by: a good cover story, nonreactive measures
 

Experimenter Bias

Problems with self-fulfilling prophecies, biased perceptions, and even flat-out cheating

Avoid by: naïve or blind experimenters
 

Maintaining Internal Validity

random assignment to conditions!!!

standardization across conditions

pilot testing

deception

naïve/blind/mechanical experimenters

repeated-measures designs with baseline
 
 

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