Probabilidade ou chance

Introduction: What is the chance of a smoker and sedentary person having a stroke? Or would it be more correct to ask what the probability of stroke is? By taking into account the definition of probability in Portuguese-language dictionaries, one has the probability that the favorable perspective of something will occur; possibility, chance. So probability and chance would be synonymous. However, when recourse to epidemiology and statistics, it is perceived that probability and chance have different meanings. The purpose of this text is to discuss the difference between probability and chance. In order to do so, we will approach concepts about analytical population epidemiological studies and epidemiological measures (Prevalence Ratio, Relative Risk and Odds Ratio), depending on the epidemiological measure, the chance or the probability will be calculated. In the epidemiological study of the case-control type, the Odds Ratio is calculated and in the cohort and experimental studies the Relative Risk is calculated. In the cross-sectional study, the Prevalence Ratio is usually calculated. The epidemiological measure Odds Ratio is the only one that estimates the chance of occurrence of an event, since it is the rate of the expected and unexpected, the others, Prevalence Ratio and Relative Risk, estimate the probability of occurrence of an event, since they estimate the events which occurred among the possible.