PRINCIPLES OF TOXICOLOGY - Biology East Borneo

21.8 MEASUREMENT OF ASSOCIATION OR RISK 517time. The incidence rate is the classic measure of disease rates in an epidemiologic study. For example,if four out of the five lung cancer cases developed in the asbestos-exposed group of workers discussedabove over the course of one year, then the incidence rate for lung cancer in that worker populationwould be 4/1000 per year or 0.4. The incidence rate is different from the prevalence since only personswho develop a disease during a set period of time are included. Therefore, incidence rates andprevalence cannot be directly compared. Incidence rates are the measure of disease frequency in cohortstudies.21.8 MEASUREMENT OF ASSOCIATION OR RISKIn epidemiologic studies, populations with different frequencies of disease or exposure are oftencompared to suggest or establish an association between particular diseases and exposure. This is alsodescribed as the risk of disease from a particular exposure. For example, if the incidence rate of lungcancer in a worker population without asbestos exposure is 1/1000 workers per year, this can becompared to the rate of lung cancer in an asbestos exposed population of 4/1000 per year. Thecomparison shows that the rate of lung cancer is higher in the asbestos-exposed population than theunexposed population. This suggests an association between asbestos exposure and lung cancer, orthat there is a risk of lung cancer with asbestos exposure.As mentioned above, prevalence is the measurement of disease or exposure frequency incross-sectional studies. The risk of disease can be examined using the prevalences of disease in theexposed and unexposed populations by formulating the rate ratio and the rate difference (Table 21.3).The rate ratio is the ratio of the prevalence of a particular disease in the exposed population to theprevalence of the unexposed population. The rate difference is the difference between the prevalenceof a particular disease between the exposed population and the unexposed population. A rate ratiogreater than or less than one indicates increased or decreased risk of disease; a rate difference greaterthan 0 indicates increased risk of disease. These measures of risk can be tested for statistical significanceusing the chi-square (χ 2 ) test and a confidence interval.The incidence rate is the measurement of disease or exposure frequency in cohort studies. The riskof disease in the exposed and unexposed population is examined with incidence rates of disease in theexposed and unexposed populations using the rate ratio and the rate difference (Table 21.4). The rateratio is the ratio of the incidence of a disease in the exposed population to the incidence in the unexposedpopulation. The rate difference is the difference between the incidence of a particular disease betweenthe exposed population and the unexposed population. These measures of risk can also be tested forstatistical significance using the χ 2 test and a confidence interval.Rate ratios are often standardized to create a standardized rate ratio (SRR) so that rates fromdifferent populations are compared after eliminating confounding (see text below). For example, ifone of the populations is older, then age as a confounder can lead to a false conclusion when comparingcancer disease rates in the two populations; the older population will inevitably have a higher cancerTABLE 21.3 Prevalence Data: Rate Ratio and Rate DifferenceDisease+ –+ A BExposure– C DPrevalence of disease in exposed population = A/(A + B)Prevalence of disease in the unexposed population = C/(C + D)Rate ratio = A/(A + B) divided by C/(C + D)Rate difference = A(A + B) – C/(C + D)