640 Obstetrics and Gynecology Board Review •••
❍ Define attack rate of a disease outbreak.
It is the number of persons affected by the disease among the persons at risk of the disease.
It is calculated as follows:
Attack rate (AR) = (number of new cases/persons at risk) × 100
❍ What are the two primary modes of transmission of a disease outbreak?
Disease can be spread person-to-person and by common sources of exposure (contact with a risk factor originating in a
shared environment of people).
❍ When should a disease outbreak be investigated?
Consideration should be given to the number and severity of affected persons, an unknown, cause and public
health concerns.
❍ What is the term used for the time duration from diagnosis to death?
Survival time; the median survival time is the duration of time from diagnosis to death that is exceeded by 50% of
subjects with a particular disease.
❍ How is “survival” defined in epidemiologic terms?
Survival is the likelihood of remaining alive for a specified period of time after the diagnosis of a particular disease.
It can be estimated as follows:
S = A-D/A where S is survival, A is the number of newly diagnosed patients under observation, and D is the
number of deaths observed in a specified period of time. It can be expressed as a decimal or converted to the
corresponding percentage.
❍ How do you define premature death?
Premature death measures the years of potential life lost to a particular disease.
❍ Define descriptive epidemiology.
Descriptive epidemiology includes activities related to characterizing the distribution of diseases in a population.
❍ What is disease frequency?
Disease frequency is the amount of disease or morbidity in a population expressed as incidence or prevalence.
❍ What types of measures are used to describe disease frequency?
Incidence rate, prevalence, and risk are used to describe disease frequency.
Incidence rate measures how rapid newly affected cases of the disease of interest develop. It is useful for tracking
changes in the occurrence of disease over time. It can be calculated as follows:
IR = A/PT, where IR is incidence rate, A is the number of new cases of the disease in a population, and PT is the
measure of net time that persons in the population at risk of developing the disease is observed. PT is also
known as person time; usually expressed as number of cases per 100,000 per year.
Prevalence is the proportion of a population that is affected by a disease or condition at a given time. It includes
new, existing, and recurring cases in a population. It can be calculated as follows:
P = C/N; where P is prevalence, C is the number of new, existing, and recurring cases at a given point in time, and
N is the total population at risk.