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External The Experimental Approach - An
Example: Rosenthal & Jacobsen
Expectancy Change in
IQ Score •“Bloomer” label assigned randomly + students
/parents unaware of label = High Internal Validity
•Sample of varied ethnicity, ability etc. + Real
classroom, real IQ tests = High External Validity
External Did Rosenthal & Jacobsen
prove SFP to be true?
• Could this outcome be due to chance
variation and not the effect of the IV?
What is the likelihood that this outcome is
due to chance?
If it’s < 5% (i.e., p < .05), we decide that the
result is statistically significant
What factors affect significance?
• Magnitude of effect
• Variability in data
• Sample size We NEVER prove anything in
psychological research because We study samples, not populations
We draw inferences about populations
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