number of cases of a disease or other health phenomenon being studied.
• single cases may have public health significance
─Case reports of patients with particularly unusual presentations or combinations of symptoms often spur epidemiologic investigations.
─Ex one case of Ebola virus
occurrence of new disease or mortality within a defined period of observation (time period) in a specified population.
Population at Risk
members of the population who are capable of developing the disease or condition being studied are known as the population at risk.
A rate formed by dividing the number of new cases that occur during a time period by the number of individuals in the population at risk
Number of existing cases of a disease or health condition, or deaths in a population at some designated time.
•Variations: Point prevalence and Period prevalence/Lifetime prevalence
•All cases of a disease, health condition, or deaths that exist at a particular point in time relative to a specific population from which the cases are derived.
Point prevalence = Number of persons ill at a point in time
Total number in the group
All cases of a disease within a period of time
Cases diagnosed at any time during the person’s lifetime
•Ex: Lifetime asthma diagnosis
Interrelationships Between Incidence and Prevalence
Factors that cause prevalence to increase:
─Increase in incidence
─Longer duration of the case
─In-migration of cases
─Prolongation of life of patients without a cure
•Factors that cause prevalence to decrease:
─Decrease in incidence
─Shorter duration of disease
─In-migration of healthy people
─Improved cure rate of disease
•A type of rate that has not been modified to take account of any of the factors such as the demographic make-up of the population that may affect the observed rate
•Includes a time period during which an event occurred.
•Numerator consists of the frequency of a disease over a specified period of time.
•Denominator is a unit size of population.Aid in making comparisons but have limitations
Crude Death Rate
•The crude death rate is a type of crude rate.
•Can be expressed in terms of any unit size of a population that is convenient.
─For example, infant mortality is expressed per 1,000 live births.
•The population from which cases of a disease have been taken
•Ex: calculation of the annual crude death rate in the United States
Case Fatality Rate (CFR)
The number of deaths due to a disease that occur among persons who are afflicted with that disease=
Number of deaths due to disease “X” Times 100during a time periodNumber of cases of disease “X”
Proportional Mortality Ratio (PMR)
The number of deaths within a population due to a specific disease or cause divided by the total number of deaths in the population
A measure that refers to mortality (or frequency of a given disease) divided by the population size at the midpoint of a time period times a multiplier.
Mortality (or frequency of a given disease) X 100,000
Population size at midpoint of time period
Age-Specific Rate (R)
•The number of cases per age group of population during a specified time period
The frequency of a disease in a gender group divided by the
total number of persons within that gender group during a
time period times a multiplier
•A rate of morbidity or mortality in a population in which statistical procedures have been applied to permit fair comparisons across populations by removing the effect of differences in the composition of various populations