**Background**

This is a confusing area but one that must not be ignored especially with so many claims being made about the efficacy of different treatments or diagnostic techniques.

Case control studies-Utilized when an outcome is recognized and the exploration of a suspected causative agent is at an early stage. This is useful if there is a very long time lag between exposure and outcome or if the outcome of interest is very rare. Patients with an outcome of interest are compared with control patients without the outcome. The association between exposure and outcome is theodds ratioin case control studies. In general, a relative risk should not be used to express results in this type of study because the disease prevalence is not known and the apparent relative risk is dependent upon the number of controls chosen.

Cohort study-A group of individuals are exposed to an agent and compared to a control group who are not exposed. Both groups are observed until an event of interest occurs or for a prespecified time period. The association of exposure and outcome is therelative risk.

Confounding effect-This is lack of comparability between groups. It should be adjusted to allow valid inferences on the effectiveness of the intervention. When a random allocation is carried out, 2 or more random subsamples of the study population are created. If the sample size is large, the may increase the comparability of the treatment and control groups with regard to extraneous or confounding variables.

Incidence-Extent or frequency of occurrence

Interactions-Indicates the benefits of intervention are exclusively to or greatest for individuals possessing special susceptibility factors.

Odds ratio-This is the ratio of the number of events to the number of nonevents. The odds are equal to the probability divided by 1 minus the probability. An event is defined by the presence of an exposure to a suspected causal agent or risk factor in a case-control study. The odds of exposure of cases are divided by the odds of exposure among control to derive the odds ratio. The odds ratio is a reasonable approximation of the relative risk when the outcome is relatively large (eg, when less than 1% of the people exposed to an agent develop disease). The odds ratio produces larger errors as the outcome rate rises above 1%

Odds Ratio95% confidence interval does not include 1Interpretation1Yes No association >1Yes Positive association between exposure and outcome at the 5% significance level (the odds of exposure is greater in cases than in controls) <1Yes Negative association between exposure and outcome at the 5% significance level (the odds of exposure is smaller in cases than controls) 95% confidence interval does include 1 Association of exposure and outcome is not proved by the study at the 5% significance level

Prevalence-The total number of cases of a disease in a given population at a specific time.

Risk, Absolute (AR)-Likelihood of an individual or group of patients developing a disease process within a given period or time, expressed as a percentage.

Risk, Relative(RR)-An assesment of the association between exposure and outcome in a cohort study.

Relative Risk95% confidence interval does not include 1Interpretation1Yes No association >1Yes Positive association between exposure and outcome at the 5% significance level (outcome is more likely in the exposed cohort) <1Yes Negative association between exposure and outcome at the 5% significance level (outcome is less likely in the exposed cohort) 95% confidence interval includes the relative risk of 1 Association of exposure and outcome is not proved by the study at the 5% significance level

Epidemiology: Beyond the Basics.Aspen Publishers Inc. 2000.

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Last Updated 1/16/2002

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