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Ex Post Facto research

Ex Post Facto research

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Ex Post Facto research

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  1. Ex Post Facto research • Researcher cannot manipulate some variables and therefor selects participants that have certain values for those variables by themselves (gender, personality, illness, ...) • Pros • Often only possible approach • Cons • Selection threat to internal validity: probably not only independent variable differs among participants

  2. Ex Post Facto - example Depression in rape victims

  3. Research design X1 = rape victim X2 = control Yij = average depression score in group i, at time j ---- indicates possible unequality of groups in both conditions

  4. Generations TIME Timespan of research Longitudinal design

  5. Assess evolution from longitudinal study • Comparison of successive measures of Yi • Experimental loss • Cost • Insensitive to cohort effects

  6. Generations TIME Timespan of research Cross-sectional research

  7. Assess evolution from cross-sectional study • Comparing different measures of Yi(from different groups) • No experimental loss • Fast and cheap • Cohort effects occur but cannot be assessed

  8. Generations TIME Timespan of research Cohort-sequential research

  9. Assess evolution from cohorte-sequential study • Yij = measure at time i in participants of age j • Evolution within cohort is assessed from successive measures Yij • Evolution between cohorts (apart from age) is assessed from comparison between Y1N and Y2N • Experimental loss • Cost • Cohort effects can be assessed

  10. Longitudinal <> Cross-sectional research Intelligence and age Longitudinal Cross-sectional Average IQ Age