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Probability is widely applied to measure risk associated with disease and drug therapy. Risk factor is a condition or behavior that increases the chance of developing disease in a healthy subject over a period of time. An example could be the risk of developing lung cancer over time with tobacco smoking. Another example of risk factor is the possibility of developing hearing loss after receiving an aminoglycoside antibiotic for a period of time.

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The term relative risk (RR) or risk ratio is the most frequently used probability term to measure association of risk with exposure (to a drug or a behavior). Risk factors may be genetic, environmental, or behavioral. They have important implications in both pharmacokinetics and drug therapy (see Chapter 12). Risk factors may be casual or merely a marker that increases the probability of a disease.

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Often risk information is collected in a controlled manner over a period of time by survey, either from the past or forward in time. In a prospective cohort study (also known as a cohort study, prospective, follow-up, or longitudinal study), a cohort of healthy subjects exposed to different levels of a suspected risk is followed forward in time to determine the incidence of risk in each group. For example, in a hypothetical study, the risk of thrombophlebitis was studied in a group of randomly selected women: 500 women taking and 500 matched women not taking a birth control pill for 10 years. The RR of 10 for thrombophlebitis was calculated from a risk of thrombophlebitis in women exposed to the birth control pill versus women not exposed to the drug using a dichotomous 2 × 2 table as shown in Table A-1, where A and B are the number of subjects who developed thrombophlebitis. The probability of exposed in this case is A/(A + B) and that of not exposed is C/(C + D). In this case, assume the exposed risk is 0.025 and the not-exposed risk is 0.0025. Then RR = 0.025/0.0025 = 10. Thus, the relative risk of thrombophlebitis in women on the birth control pill for 10 years is 10 for this group of women studied.

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Table A-1 Tabulation from a Hypothetical Cohort Study of Female Subjects on the Pill—with and Without Developing Thrombophlebitis for 10 Years
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A second method of studying risk is the historical or retrospective cohort study, which looks backward in time to determine the present risk. The cohort of exposed ...

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