Epidemiology

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Central Tendency: The middle of a distribution. Described by mean, median, and mode.

Mean: The arithmetic average in a set of values. The average.

Median: For a set of values arranged in order of magnitude, the median is the middle value for odd numbers of values and the average of the two middle values for an even number of values.

Mode: For a set of values, the mode is the value that occurs most often.

Incidence Rate: Number of new cases of a disease in a specified period / average population during that period. Rate is usually expressed as per 100,000.

Incidence Rate: Number of new cases of a disease in a specified period / average population during that period. Rate is usually expressed as per 100,000.

Prevalence Rate: Number of people with a disease at a given point (period)/ population at risk at a particular point (period). Rate is usually expressed as per 100,000. Prevalence = Incidence X duration

Point prevalence = at a specific point in time.

Period prevalence = during a specific period of time.

Mortality Rate: Total number of deaths / total number of people at risk per unit of time.

Standardized Mortality Rate: Mortality rate adjusted for a confounding variable such as age.

Case Fatality Rate: The rate of death from a disease in patients with that disease. For example, 1000 males in Charleston, SC are infected HIV. During 1998, 10 HIV-infected males in Charleston, SC died for a case fatality rate of 10/1000 or 1%.

Attributable Risk: Additional risk of disease is the exposed group over that in the unexposed group. If lung cancer occurs in 1 of 10,000 nonsmokers every year (made up numbers) and in 100 of 10,000 smokers, the attributable risk of smoking is 99 of 10,000. Attributable risk = Incidence in exposed minus incidence in nonexposed.