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Traps and pitfalls in medical statistics Arvid Sjölander

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Traps and pitfalls in medical statistics Arvid Sjlander Slide 2 26 april 2015Arvid Sjlander2 Motivating example You are involved in a project to find out if snus causes ulcer. A questionnaire is sent out to 300 randomly chosen subjects. 200 subjects respond: We can use the relative risk (RR) to measure the association between snus and ulcer: Can we safely conclude that snus prevents ulcer? Ulcer YesNoR Snus Yes228 2/30 0.07 No1715317/170=0.1 Slide 3 26 april 2015Arvid Sjlander3 Outline Systematic errors Selection bias Confounding Randomization Reverse causation Random errors Confidence interval P-value Hypothesis test Significance level Power Slide 4 26 april 2015Arvid Sjlander4 One possible explanation It is a wide spread hypothesis that snus causes ulcer. Snus users who develop ulcer may therefore feel somewhat guilty, and may therefore be reluctant to participate in the study Hence, RR

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