Presentation on theme: "Critical Appraisal. Evidence –Based Medicine Formulate problem Track down best evidence Appraise the evidence critically Implement results in clinical."— Presentation transcript:
“Opportunistic” [Fishing expeditions] Pore over data until a ‘significant’ association is found Devise a biologically plausible hypothesis to fit the association
Study of parents’ occupational exposures as a risk factor for birth defects in their offspring Seven major categories of occupation identified No significant relationship (for mothers or fathers)
Study of parents’ occupational exposures as a risk factor for birth defects in their offspring Categories split into: 64 separate occupations for mother 80 for fathers 5 significant relationships found
“Procrustean” Deciding on the hypothesis to be proved and matching the data to fit the hypothesis ie. selective reporting [Procrustes, a robber in Greek mythology, made all his victims fit the length of his bed by stretching or cutting off their legs]
Exposure redefined to strengthen the association In a study of adverse effects of oral contraceptives on the outcome of pregnancy Exposure defined as OC used within 600 days before a delivery or miscarriage
Exposure redefined to strengthen the association Why 600 days? Why not 365 days (ie. 1 year)? Or 2 years? Or 18 months? Or 500 days?........
Study by Western Electric Company to assess effects of illumination on production at the Hawthorne plant in Chicago Control group – worked under constant illumination Experimental group – worked under varying illumination
Hawthorne Effect Production increased in experimental and control groups at same rate
Hawthorne Effect Production increased in experimental and control groups at same rate Increases in production were caused by the increased attention that workers received from management
Regression to the Mean Sir Francis Galton’s publication: “Regression towards mediocrity in hereditary stature” Very tall parents tend to have children who are not quite as tall, while short parents tend to have taller children
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