Medical statistics at SUN

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1 02/11/2014 Medical statistics at SUN III I.C. Methods in Epidemiology Ciro Gallo Dip.to di Salute mentale e fisica e Medicina preventiva v. L. Armanni ciro.gallo@unina2.it Arrangement to meet by phone or Textbook 1

2 02/11/2014 If you like more 2

3 Methods in Epidemiology: Educational targets General target At the end of the course students should be able: To use epidemiological and bio-statistical tools for a better understanding of biomedical information and for improving medical decision making Methods in Epidemiology: Educational targets Intermediate targets At the end of the course students should be able to: To report how disease frequency may be measured at community level To illustrate implications for health of ageing in Western countries To use main statistical methods to report biomedical information To use main statistical methods to infer conclusions from research findings To detail strengths and weaknesses of main epidemiological designs to compare measures of disease occurrence To apply main statistical methods to evaluate prognosis To apply main statistical methods to evaluate diagnostic accuracy To apply main statistical methods to evaluate treatment efficacy To critically appraise Evidence-Based Medicine (EBM) 3

4 Methods in Epidemiology: Educational targets Just some workable targets At the end of the course students should be able to: o To differentiate between internal and external validity o To explain the meaning of probability o To differentiate the related concepts of parameter and estimate o To explain the meaning of sample distribution o To differentiate between standard deviation and standard error o To illustrate the principles of statistical inference o To interpret confidence intervals o To interpret statistical tests o To differentiate errors α and β o To differentiate main tests to compare two or more groups o To illustrate criteria for choosing the appropriate statistical test I (doctors) II (midwives) Births Dead Mortality rate 9,92 3,38 Mortality rates (%) of women due to fever after birth in two maternity clinics in Vienna ( ) Ignaz Semmelweis ( ) WHY? 4

5 HYPOTHESES Epidemic influences Crowding Food Courses of treatment Lesions due to students Labour modalities Plague is not selective Women giving birth outside clinics had lower mortality rates Greater in the second clinic Similar in the two clinics Similar in the two clinics Mortality rates did not lower when less students were admitted Changed but mortality rates did not lower In 1847 Kolletchka, a friend of Semmelweiss, was injured by a scalpel of a student during authopsy Kolletchka eventually died presenting symptoms that Semmelweiss recognized as similar to those of women dying after birth. Semmelweis hypothesized that Kolletchka was poisoned by an unknown substance released to the scalpel by the corpse during authopsy Semmelweis hypothesized a similar path for women who were examined by students immediately after authopsies without washing, or just superficially washing, their hands 5

6 Semmelweis tested his hypothesis. Before examining women students had to wash their hands in a solution with a sanitizer I (doctors) II (midwives) ,27 1,33 Please mind that midwives did not perfrom authopsies women giving birth outside clinics were not examined Semmelweis findings were strongly opposed by Academy Semmelweis dead of septicemia in 1865 in madhouse What can we learn from Semmelweis story? To describe only is not enough To give right answers both intuition and method are needed to formulate appropriate hypotheses and to verify them Semmelweis was not at all the first who raised the question He formulated several hypotheses according to the day knowledge (most were wrong) He planned a simple experiment to test his hypothesis 6

7 The scientific method Clinics Clinical problem Science Observation Generating hypothesis Generating hypothesis Diagnostic tools Testing hypothesis Tentative diagnosis Conclusion When studying methods we deal with rules and criteria, rather than contents, of medical approach C.Scandellari 2009 Director of clinics at the Charité of Paris. He first introduced the statistical (or quantitative) method when studying diseases, that is disease knowledge on natural history, or clinical presentation or treatment, can only advance from the analysis of group of patients 7

8 Common sense is usually misleading Il buon senso c era, ma se ne stava nascosto, per paura del senso comune A. Manzoni. I promessi sposi. Cap. XXXII A common-sense approach to a simple problem can be overtly wrong, until we educate our common sense to appreciate better the nature of the problem. Rothman KJ. Epidemiology. An introduction. Risk of death in a 20-year period among women of Whickam, according to their smoking status at the beginning of the period Vital status Smoker Nonsmoker Total Dead Alive Total Risk(dead/total) From Rothman (2002), data from Vanderpump et al. (1995) 8

9 Age distribution of women of Whickam, according to their smoking status at the beginning of the period Age Smoker Nonsmoker Total From Rothman (2002), data from Vanderpump et al. (1995) Age distribution of women of Whickam, according to their smoking status at the beginning of the period Age Smoker(%) Nonsmoker(%) Total (%) (9.5) 62 (10.7) (21.3) 157 (27.0) (18.7) 121 (20.8) (22.3) 78 (13.4) (19.8) 121 (20.8) (6.2) 129 (22.2) (2.2) 64 (11.0) 77 From Rothman (2002), data from Vanderpump et al. (1995) 9

10 Risk of death in a 20-year period among women of Whickam, according to their smoking status and age Age Smoker(%) Nonsmoker(%) Total (%) Overall From Rothman (2002), data from Vanderpump et al. (1995) Simpson s paradox 10

11 Skepticemia Is a condition uncommon and scarcely contagious. Medical schools usually favor a permanent immunization against it Skrabanek P, McCormick J (1989) Follies and Fallacies in Medicine 11