Epi Measures of Disease Occurence

Ratio

• Value obtained by dividing one quantity by another

• Ratio = x/y

• There does not have to be a relation between two values

Proportion

• Type of ratio

• Numerator is part of the denominator

• Proportion = A / A + B

Percentage

• A proportion that has been multiplied by 100

• Percentage = (A / A + B) * 100

What’s the point?

• tells u how important a health outcome is in relation to the size of a group

• Proportion is 10%, shows a different picture than if the proportion is 80%

• Very important in epidemiology to get an accurate denominator

Rate

• Type of ratio, but it involves time

• Rate = number of events/time

• Measures:

  • Risks associated with exposure

  • Speed of a development of a disease

• Can be used to make comparisons between populations

Epidemiological measures: Count

• Number of cases of a disease or other health phenomenon being studied

• Also called “frequency”

Epidemiological measures: Incidence

• Number of instances of illness commencing, or of persons falling ill, during a given period in a specified population

• More generally, the number of NEW health-related events in a defined population within a specified period of time

What do measures of incidence do?

• Measure

  • Risk of acquiring a disease

  • Rate at which new cases of disease develop in a population

  • Other health outcomes

• Important to the study of causal mechanisms or causes of diseases

Incidence Rate

Risk

# of new cases of disease (over a specific time period) / # of people at risk during the time period

Rate

# of new cases of disease (over a specific time period) / amount of person time at risk for disease

Cumulative Incidence (ris)

• Incidence rate, but when all individuals in the population are at risk throughout the time period of observation

• Example: participants in a cohort study and no new members are allowed to enter the study after it begins

• The population is static

Incidence Density

• Incidence rate, but when the time periods of observation of the members of a population vary from person to person

• Example: a cohort study where participants observed for varying periods of time because some drop out before the study is completed

• Person-time = total period of time that each individual at risk has been observed

Attack Rate

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

• Often related to a specific exposure

• Describes foodborne illness, infectious disease, and other acute epidemics

• Not a true rate because the time dimension is often uncertain

Prevalence (LOOK AT SLIDES FOR EXAMPLE)

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

• Used to measure the scope and distribution of health outcomes in the population (“burden of disease”)

• Expressed as a proportion

• Two types:

  • Point prevalence

  • Period prevalence

Point 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

• Expressed as a proportion

  • Proportion x 100 is %

Period Prevalence

The total number of cases of a disease that exist during a specific period of time (week, month, year)

Factors influencing prevalence

Measures of Mortality

• Life expectancy

• Crude death rate

• Crude fatality rate

• Proportional mortality rate

Life expectancy

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

• In 2013, the life expectancy for the US population was 78.8 years overall

  • 81.2 years for females

  • 76.4 years for males

Crude rate

• type of rate that has not been modified to take into account any other factors that may affect the rate

  • examples: crude death rate

Specific rate

• type of rate referring to a particular subgroup of the population

  • examples: cause-specific, age-specific

Adjusted rate

• type of rate in which statistical procedures have been applied to permit fair comparisons across populations

  • examples: age-adjusted rate

Benefits of Rates

• Improve our ability to make comparisons

• Example: Compare deaths in Texas to deaths in Rhode Island in 2020…

  • Deaths in Texas: 249,267

  • Deaths in Rhode Island: 12,110

  • The size of the population is crucial to understanding the full picture!

Crude death rate

Case fatality rate (CFR)

Proportional mortality ratio (PMR)

Limitation of Rates

• Reduces the standard of comparison to a common denominator

• Example: Compare deaths due to heart disease between Texas and New York in 2013

  • Death rate in TX: 152 per 100,000

  • Death rate in NY: 224.1 per 100,000

  • There may be differences in the population (age) we need to consider…

Cause specific rate

Age-specific rate

Sex-specific rates

Adjusted rates

• 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

Age-adjusted rates

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

Direct method rates

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

Measures of natality: Maternal mortality rate

• reflects health disparities such as healthcare access

Measures of natality: Fetal death rate/late fetal death rate

Measures risk of death of the fetus

Measures of natality: General fertility rate

compares populations and subgroups regarding their fertility

Measures of natality: Crude birth rate

to project population changes

Maternal mortality rate

Infant mortality rate

Fetal mortality rate

Crude birth rate

General fertility rate

Perinatal mortality rate

Incidence measures

new cases

Crude death rate (mortality rate)