Ch 3 Epidemiology - Measures of Disease Occurence

quantitative variables

• ratio

  • ex: 5:1 male:female

• proportion

  • male/ male+female

• percentages

  • male/ male+female x 100%

• rate

  • the number of cases of a disease/ some factor of change in time

• count

  • the number of cases that have happened

why are ratio, proportion, & percentages important to public health?

• it helps identify high risk populations

• key functions for epidemiologists

• comparison for factors behind disease

• get a profile on the disease (90% vs 10%)

why are rate & count important to public health

• measures the risk associated with exposures

• helps measure how fast a disease is spreading & compares between populations

incidence

• the number of new cases of a disease, health related events in a population in a given period of time, “incident”

different types of incidence measures

• risk of acquiring a disease

• incidence rate

• incidence density

  • cumulative rate

• attack rate

incidence rate

• (the number of new cases/ population at a point in time) X 100,000

incidence density

• when the time periods of observation of the members of a population vary from person to person

• the number of new cases/ total percent - time of observation

attack rate

• when the occurrence of disease among a population at risk increases greatly over a short period of time

• measure that is used for a single incidence for a particular disease

• iII/(iII + well) x 100

cumulative incidence

• when all individuals in the population are at risk throughout the time period of observation

• number of new cases over a time period/ total population at risk during the same time period

point prevalence

• number of existing cases of a disease or health condition, or deaths in a population at some designated time

• number of persons ill/ total number in the group

period prevalence

• the total number of cases of a disease that exist during a specified period of time (e.g. a week, month, year, or other interval)

lifetime prevalence

• cases of disease diagnosed at any time during the person’s lifetime

factors that increase observed prevalence

• longer duration of the disease

• prolongation of life of patients without cure

• increase in new cases (increase in incidence)

• in-migration of cases

• out-migration of healthy people

• in-migration of susceptible people

• improved diagnostic facilities

factors that decrease observed prevalence

• shorter duration of disease

• high case-fatality rate from disease

• decrease in new cases (decrease in incidence) in-migration of healthy people

• out-migration of cases

• improved cure rate of cases

relationship between incidence and prevalence

• the relationship between the two depends mainly on how long the disease stays between contracting and cure/death

• P=IRxD

• P = prevalence

• IR = incidence rate

• D = average duration of disease

life expectancy

• the number of years that a person is expected to live, at any particular year

life expectancy at birth

• the average number of years that a group of infants would live if the group was to experience throughout life the age-specific death rates present in the year of birth

types of rates

• crude death rate

• case fatality rate

• proportional mortality ratio

• cause specific rate

• age specific rate

• sex specific rate

• adjusted rates

crude death rate (CDR)

• (number of deaths in a given year/ reference population (midpoint) ) x 100,000

case fatality rate (CFR)

(number of deaths due to disease “X”/ number of cases of disease “X’) x 100

proportional mortality ratio (PMR)

(mortality due to a specific cause during a period of time/ mortality due to all causes during the same time period) x 100

cause specific rate (CSR)

(mortality/ population size at midpoint of time period) x 100,000

age specific rate

(number of deaths among those age 15 to 24 years/ number of persons who are age 15 to 24 years) x 100,000

sex specific rate

(number of deaths in a sex group/ total number of persons in the sex group) x 100,000

adjusted rates

• statistical procedures have been applied to permit fair comparisons across populations by removing the effect of differences in the composition of various populations

• age adjusted

  • use detailed information about age structure of population for which the rates are being age adjusted

• direct method

  • multiplying the age-specific rates for each subgroup of a population to be standardized by the number in a comparable subgroup of a standard population

maternal mortality rate

number of deaths assigned to causes related to childbirth/ number of live births x 100,000

infant mortality rate

number of infant deaths among infants age 0-365/ number of live births during the year x 1,000

fetal mortality rate

• fetal death rate= number of fetal deaths after 20 weeks or more gestation/ number of live births + number of fetal deaths after 20 weeks or more gestation x 1,000

• late fetal death rate = number of fetal deaths after 28 weeks or more gestation/ number of live births + number of fetal deaths after 28 weeks or more gestation x 1,000

crude birth rate

number of live births within a given period/ population size at the middle of that period x 1,000

general fertility rate

number of live births with in a year/ number of women age 15 to 44 years x 1,000

perinatal mortality rate

number of late fetal deaths (after 28 weeks or more gestation) + infant deaths within 7 days of birth/ number of live births + number of late fetal deaths x 1,000