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Lecture

PSYCH291 Lecture Notes - Internal Validity, Random Assignment


Department
Psychology
Course Code
PSYCH291
Professor
Joanne Wood

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True Experiments
Possible threats to internal validity
OGPTPT DESIGN
*Is selection ruled out as threat to internal validity?
Is ambiguity about causal direction ruled out as threat to internal validity?
*Is local history ruled out as threat
to internal validity?
History vs. local history
*History—some external event other than the X
*Local history—some external event other than the X (or not X) that occurs in one group
and not the other
*So, by definition, history is some external event that occurs to most people in the
sample
*Local history is something that occurs to one group of people in sample
“typical” correlational design
*Is selection a threat to internal validity?
*E.g., could people who are higher in novel, exciting activities differ from people
who are lower in novel, exciting activities on some other preexisting
characteristic that may explain the result?
*Is ambiguity about causal direction a threat to internal validity?
*Is local history a threat to internal validity? E.g., Could some external event have
occurred for those higher in novel, exciting activities but not those lower in novel,
exciting activities, or vice versa?
What’s so great about true experiments?
To illustrate how true expts rule out threats to internal validity
*Use example from research by Aron et al.
*Experimental study
*2 imaginary studies involving same two variables
*One-group pretest-posttest design
*Threats to internal validity
*Would a true expt would rule out these threats?
*Static group design
*Threats to internal validity
*Would a true expt would rule out these threats?
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