Understanding The Basic Principles Of Epidemiology

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Epidemiological studies identify associations between risk factors and disease outcomes. Identifying these associations can be useful for classifying individuals into risk sets whether or not a causal relationship between the disease and its correlate can be established. Measures of risk typically include the absolute risk, relative risk, and odds ratio. These three risk measures are described briefly below, but much more detail on them can be found in standard epidemiological reference texts such as those by Rothman46 and Selvin.47

Absolute risk is the most easily interpreted risk measure. It is usually specified in terms of risk per time unit per individual. Using the cells in Table 5.2, the absolute risk in individuals exposed to a risk factor per time unit is (a)/(a+c). An example for this type of analysis was conducted using the population-based (n = 1387) Longitudinal Study of Aging Danish Twins. The authors assessed the influence of physical activity as a risk factor for incident low back pain among seniors aged 70-100 years. They found that active lifestyle protects against incident low back pain. Absolute risk estimates for incident low back pain among participants with below average strength score engaged in strenuous physical activity and those with below average strength score not engaged in strenuous physical activity were 10 and

21 percent, respectively.48

The relative risk is the risk of disease in individuals who have been exposed to a risk factor divided by the risk of disease in individuals who have not been exposed to that risk factor. Again using the cells in Table 5.2, the relative risk in individuals exposed to a risk factor is ((a)/(a+c))/((b)/(b+d)). The relative risk for developing

Table 5.2 Parameters for calculating risk in epidemiological studies.

chronic pain from work-related stress was assessed using longitudinal data from the National Population Health Survey in Canada (n = 6571). The investigators found that relative risk for developing chronic pain was 1.39 (95 percent CI = 1.01-1.91) for medium stress and 1.80 (95 percent CI = 1.28-2.52) for high stress.49

Finally, the odds ratio is the ratio of the probability of exposure to a risk factor in diseased patients to the probability of exposure to a risk factor in nondiseased patients divided by the probability of nonexposure to a risk factor in diseased patients divided by the probability of nonexposure to a risk factor in nondiseased patients. Using the cells in Table 5.2, this ratio is ((a)/(c))/ ((b)/(d)).

When the prevalence of cases of pain in a population is low, the odds ratio approximates the relative risk (i.e. a+cffid and b+dffid). For relative risks greater than 1, the odds ratio overestimates the relative risk; however, the odds ratio is nearly unbiased when studying uncommon events such as the risk of neuropathic pain. The odds ratio is a measure used to estimate the relative risk for several reasons. For example, in case-control studies, if a disease is uncommon, the odds ratio gives a nearly unbiased estimate of its relative risk. Also, commonly used data analysis tools, such as logistic regression, readily provide estimates of odds ratios.

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