34731 - Biostatistics course Part 13 Effect measures in 2 x...

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Unformatted text preview: Biostatistics course Part 13 Effect measures in 2 x 2 tables Dr. Sc. Nicolas Padilla Raygoza Department of Nursing and Obstetrics Division Health Sciences and Engineering University of Guanajuato Campus Celaya-Salvatierra Biosketch Medical Doctor by University Autonomous of Guadalajara. Pediatrician by the Mexican Council of Certification on Pediatrics. Postgraduate Diploma on Epidemiology, London School of Hygiene and Tropical Medicine, University of London. Master Sciences with aim in Epidemiology, Atlantic International University. Doctorate Sciences with aim in Epidemiology, Atlantic International University. Associated Professor B, Department of Nursing and Obstetrics, Division of Health Sciences and Engineering, University of Guanajuato, Campus Celaya Salvatierra, Mexico. padillawarm@gmail.com Competencies The reader will obtain Risk Ratio or Odds Ratio from a 2 x 2 table. He (she) will calculate 95% confidence interval from RR or OR. He (she) will identify potential confounders and/or interactions. He (she) will apply Mantel Haenzsel test for RR, OR and Chi-squared. Introduction In part 12 of the course, we tested the association between two categorical variables. Now, we review the methods used to measure the association. We will work with binary variables, so we will use 2 x 2 tables. Example A nurse in a poor area of Mexico, was informed that many area children attending the nursery were sick of respiratory infections. She designed a cohort study to investigate the problem. During the following years 1000 children were followed. The main research question was: Attending nursery is associated with respiratory infection? Example Respiratory infection Respiratory infection Total Attending nursery Yes n % No n % Yes 37 33.9 72 66.1 109 No 43 4.8 848 95.2 891 Total 80 8 920 92 1000 Risk Ratio (RR) In health research, the term "risk" is used instead of proportion. For example: The risk of infection among children attending day care was 33.9%. Thus, the risk ratio is the ratio of two proportions. The risk of respiratory infection for those attending the nursery 37 / (37 + 72) = 37/109 = 0.339 The risk of respiratory infection in children not attending day care is: 43 / (43 + 848) = 43/891 = 0.048. The risk ratio (RR) is the ratio of these two risks. Risk ratio = 0.339 / 0.048 = 7.06 Risk Ratio (RR) In general, the risk ratio can be obtained with the following formula, where a, b, c and d are the frequencies in the 2 x 2 table. Outcome Outcome Total Exposure Yes No Yes a b a + b No c d c + d Total a + c b + d N Risk Ratio = (a /a+b) / (c/c + d) Odds Ratio (OR) The Odds Ratio (OR) is the ratio of the chance (probability) of the results between those exposed and the chance of the outcome among non-exposed....
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This note was uploaded on 02/23/2012 for the course STAT 312 taught by Professor Staff during the Fall '11 term at Rutgers.

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34731 - Biostatistics course Part 13 Effect measures in 2 x...

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