Association and Causation

Association and Causation Many diseases are the result of more than one exposure. For example, risk factors of lung cancer are exposure to smoking, asbestos, radiation or some chemical products. Public health programs to prevent disease are directed toward reducing or eliminating such causal exposures. Epidemiologic research not only focuses on the identification and assessment of risk factors but also is concerned with planning and evaluating public health interventions or control measures to reduce the incidence of disease in the population. Being able to predict the impact of removing a particular exposure on the risk of developing a disease is an important public health consideration. It allows those who are responsible for protecting the public’s health to make decisions about allocating scarce resources (time, energy, money and political capital) where they will have the most impact. It helps them answer the following questions: 1. What amount of the risk of developing a disease is attributable to a particular exposure? 2. By what percent would the risk of developing disease be reduced if the exposure were eliminated? If smoking were eliminated, what would happen to the incidence of lung cancer? Would smokers’ risk of lung cancer disappear if they stopped smoking? For public health decision-making purposes, it is valuable to be able to answer these questions from two perspectives: from the perspective of the impact of eliminating the exposure on only those who are exposed and from the perspective of the impact of eliminating the exposure on the entire population, those who are exposed and those who are not exposed. Note that for purposes of this activity risk and incidence rate (or incidence) can be considered interchangeable. Strictly speaking, however, incidence rate (or incidence) denotes the rate of new cases per unit time whereas risk denotes the rate of new cases in a fixed interval of time. Measures of Attributable Risk in the Exposed Attributable risk (AR): AR is the portion of the incidence of a disease in the exposed that is due to the exposure. It is the incidence of a disease in the exposed that would be eliminated if exposure were eliminated. The AR is calculated by subtracting the incidence in the unexposed (Iu) from the incidence in the exposed (Ie): AR=Ie – Iu Attributable risk percent (AR%): AR% is the percent of the incidence of a disease in the exposed that is due to the exposure. It is the proportion of the incidence of a disease in the exposed that would be eliminated if exposure were eliminated. The AR% is calculated by dividing the attributable risk (AR) by the incidence in the exposed (Ie) and then multiplying the product times 100 to obtain a percentage: Ie – Iu AR AR% = Ie X 100 or AR%= Ie X 100 Measures of Attributable Risk in the Population Population attributable risk (PAR): PAR is the portion of the incidence of a disease in the population (exposed and nonexposed) that is due to exposure. It is the incidence of a disease in the population that would be eliminated if exposure were eliminated. The PAR is calculated by subtracting the incidence in the unexposed (Iu) from the incidence in total population (exposed and unexposed) (Ip): PAR= Ip- Iu Population attributable risk percent (PAR%): PAR% is the percent of the incidence of a disease in the population (exposed and nonexposed) that is due to exposure. It is the percent of the incidence of a disease in the population that would be eliminated if exposure were eliminated. The PAR% is calculated by dividing the population attributable risk (PAR) by the incidence in the total population and then multiplying the product times 100 to obtain a percentage: Ip - Iu PAR PAR%= Ip X100 or PAR% = Ip X 100 Practice Activity The preventive advantages of eating fish have been reported in numerous studies. A recent cohort study1 reported that not eating fish increased the risk for stroke. The table below shows the results of this study: Eating Fish and Risk of Stroke Eating Fish Cases of Stroke Noncases of Stroke Total Almost daily(exposed) 23 (a) 779 (b) 802 Never (unexposed) 82 (c) 1,549 (d) 1,631 Total 105 2,328 2,433 Calculate: You must show your work for any credit. Answers alone will not be graded and the result will be a grade of 0 for this activity. Incidence in the exposed: Ie = a/(a+b)* 100= Incidence in the unexposed: Iu = c/(c+d) * 100 = Incidence in both combined: Ip = a+c/(a+b+c+d) * 100 = Relative risk: Ie/Iu = AR= AR%= PAR= PAR%= Interpret the above data in terms of risk of stroke and eating fish. What facts might confound these results (note: consider the exposure scale).