Dear all, The next seminar in our series of statistics talks for people interested in quantitative methods in biology and medicine will take place on *Thursday 24 October 2013 at 11h00* in the Salle Delachaux (Biopôle 2, premier étage, Route de la Corniche 10, Lausanne, M2: Vennes). The speaker will be Valentin Rousson from IUMSP/CHUV, who will speak about "On the consequences of dichotomization on correlations and odds-ratios" Abstract: It is well known that dichotomizing continuous data has the effect to decrease statistical power when the goal is to test for a statistical association between two variables. Modern researchers however are not only focusing on statistical significance, but also on an estimation of the "effect size" (i.e. the strength of association between the variables) to judge whether a significant association is also clinically relevant. In this talk, we are interested in the consequences of dichotomizing continuous data on the value of an effect size in some classical settings. It turns out that the conclusions will not be the same whether using a correlation or an odds-ratio to summarize the strength of association between the variables: whereas the value of a correlation is typically decreased by a factor sqrt(pi/2) after each dichotomization, the value of an odds-ratio is at the same time raised to the power sqrt(2). From a descriptive statistical point of view, it is thus not clear whether dichotomizing continuous data leads to a decrease or to an increase of the effect size, as illustrated using a data set to investigate the relationship between motor and intellectual functions in children and adolescents. Hope to see you there! Frédéric (for the organizers: Jérôme Goudet, Valentin Rousson, Frédéric Schütz) -- Frédéric Schütz, PhD Frederic.Schutz@isb-sib.ch Bioinformatics Core Facility (Delorenzi group) Swiss Institute of Bioinformatics http://www.isb-sib.ch/ University of Lausanne http://www.unil.ch/ Batiment Genopode Quartier Sorge Phone: +41 21 692 40 94 CH-1015 Lausanne Fax: +41 21 692 40 65