Module 1
Premature mortality
Death prior to age 75 years; Years of potential life lost (YPLL) (For each death, subtract age at death from 75 years -Then add years of life lost for all deaths from this specific cause)
Population attributable risk (PAR)
Difference between disease incidence in the total population and disease incidence in the unexposed; PAR = Cumulative Incidence in total population - Cumulative Incidence in unexposed group/ incidence in total population; can be expressed as a proportion or percent
Rate
Division of two numbers; time is always in the denominator; need to know numerator, denominator, and time; ranges 0 to infinity; Rate = a / T -a = number of people with certain characteristic in defined population -T = number of people in the population per unit length of time (person-time)
Dynamic population
open; defined by shared characteristic that is transient; adds new members through migration and births or loses members through emigration and deaths; defined by being in a state or condition ex. residents of Boston, Graduate students
Risk Ratio (RR)
Cumulative incidence; cumulative incidence among exposed/cumulative incidence among unexposed; RR = (a/ a+b)/ (c/c+d)
Rate ratio (RR)
Incidence rate; RR= (a/ PYe)/ (c/PYu); incidence rate among exposed/incidence rate among unexposed; Total person-time among exposed and unexposed
Attack rate
Most often used in outbreak investigations (e.g., food-borne disease outbreaks); Time period of observation assumed to be small (few hours or a few days)
Incidence Rate or incidence density
the occurrence of new cases of a disease arising during at-risk person-time of observation; A true rate; Assumes individuals in the denominator are followed for different lengths of time; Dynamic population; Tells you how fast disease is occurring in the population
Mortality rate (death rate)
total number of deaths from all causes per 100,000 population; usually expressed for a 1-year period
crude mortality rate
weighted average of the age-specific mortality rates; Where the weights are equal to the proportion of the total population in each age category (Not just age, any characteristics of interest)
Proportionate mortality
what proportion of all deaths was caused by one specific cause (e.g., cardiovascular disease); often as a percentage; not a measure of risk
Case-fatality rate
what proportion of individuals who have a certain disease die within a specified period after their disease begins or was diagnosed; measure of the severity of disease
Characteristics of persons
•Gender •Age •Marital status •Nativity and migration •Race/ethnicity •Socioeconomic status
CI = IR x t
•When incidence rate is constant -and cumulative incidence is small (<10%): -where CI = cumulative incidence IR = incidence rate t = time
CI = 1 - exp(-IR-t)
•When incident rate is constant -And cumulative incidence is larger, need to account for population at risk declining exponentially over time -where CI = cumulative incidence IR = incidence rate t = time
Relative Risk (RR)
Ratio of disease incidence in the exposed to disease incidence in the unexposed
Attributable risk (AR)
Risk Difference (RD) or Ratio Difference (RD) Difference between disease incidence in the exposed and disease incidence in the unexposed; AR = cumulative incidence in exposed group - cumulative incidence in unexposed group; AR = (a/a+b) - (c/c+d); can be expressed as a proportion or percent
Ratio
a comparison of two unrelated quantities using division; numerator does not have to be part of the denominator ex. sex ratio (number of males/number of females)
Relative measure of excess risk
calculate the ratio of two measures of disease frequency
Calculating incidence
closed population: cumulative incidence and/or incidence rate open population: incidence rate
Fixed population
closed; membership is permanent and defined by an event; membership is permanent ex: atomic bomb survivors, 9/11 Survivors
Case fatality rate formula
# deaths during specified time period after disease onset or diagnosis/ # individuals with specified disease; often expressed as a percentage
Proportionate mortality formula
# deaths from of certain disease during time period/ total # deaths during time period
Age-specific mortality rate formula
# of deaths from all causes among certain age group/ total # children of certain age group mid-year
Age- and cause-specific mortality rate formula
# of deaths from specific cause among children of a certain age group/ total # children of certain age group at mid-year
Prevalence formula
# of existing cases/total population
Incidence rate formula
-Numerator = number of new cases that developed during specified time period -Denominator = total person-time at risk (-Need to specify units of time (person-years, person-months, etc.)) -Need to specify time period
Cumulative incidence
-What proportion of the population became affected by the disease during the specified time period -Probability of developing disease over specified time period
Outbreak investigations
1. Formulate case definition 2. Conduct case confirmation 3. Establish the background rate of disease and find cases 4. Examine the descriptive epidemiology of the outbreak cases 5. Generate and test hypotheses about the causes of the outbreak 6. Collect and test environmental samples 7. Implement control measures 8. Disseminate information to the press and public
Direct age adjustment
Also known as direct age standardization; Apply age-specific mortality rates to a "standard" population in order to eliminate the effects of different age distributions across populations being compared
Indirect age adjustment
Also known as indirect age standardization; Used when age-specific mortality rates are not available for study population of interest; Often used to study occupational exposures; Apply age-specific mortality rates from a known population to the study population
Absolute measure of excess risk
Calculate the difference between two measures of disease frequency
Proportion
Division of two related numbers; numerator is a subset of denominator; ranges 0 to 1; often expressed as a percentage; a / a + b = a / N -a = number of people with certain characteristic in defined population -b = number of people without that characteristic in the same population -N = a + b = total population of interest
Secular time trends
Gradual changes in the frequency of disease over longer periods of time (ex. decline in heart disease mortality in the U.S.: Dietary improvements, Better treatment, Public health programs,etc.)
Burden of disease
Not just considering mortality, also incorporating quality of life; Disability-Adjusted Life Year (DALY) (Years of life lost to premature death AND years lived with a disability -Years of healthy life lost)
Standardized mortality ratio (SMR)
Observed number of deaths per year/ Expected number of deaths per year
Person-time
Only count time at risk for disease; Who is not contributing person-time: Anyone who has already developed the disease, Anyone who has left the population (Death or Lost to follow-up)
Disease distribution
Patterns of disease, according to: -Person: Who is getting the disease? -Place: Where is disease occurring? -Time: How is disease changing over time? Helps: Planning for health services; Formulating hypotheses about determinants of disease
Cyclic variations
Periodic changes in the frequency of diseases and health conditions over time; Related to changes in lifestyle of the host, seasonal climactic changes, and virulence of the infectious agent (ex. Flu season, West Nile Virus)
Analytic epidemiology
Purpose: To evaluate hypotheses about causes of disease; To evaluate the success of intervention programs Practice: Compare groups and systematically evaluation associations Examples: Experimental studies, case-control studies, cohort studies
descriptive epidemiology
Purpose: To monitor the public's health; To evaluate the success of intervention programs; To generate hypotheses about causes of disease Practice: Identify and count cases of disease in populations according to person, place, and time and conduct simple studies Examples: Case reports, case series, cross-sectional studies, ecologic studies, outbreak investigations
Disease cluster
more cases in space and/or time than would be expected (usually of a rare disease)
Cumulative incidence formula
new cases of disease that developed during the specified time period/ Denominator only includes individuals at risk for developing disease (not those who currently have disease)
Birth defect rate formula
number of babies with abnormalities at birth/ total number of live births (or total births)
Mortality rate (death rate) formula
number of deaths from all causes in that year/ total population during that year
Cause-specific mortality rate formula
number of deaths from specific cause in that year/ total population during that year
Attack rate formula
number of people who ate food who became ill/ number of people who ate food
Epidemic
occurrence of cases of disease in excess of what would normally be expected; Epidemic curve
Person-Time Estimation
person-time can be estimated by: Multiplying estimated population size at the midpoint of the specified time period by duration of time period OR Averaging the estimated population size at the start and end of the specified time period and multiplying by duration of time period OR Multiplying the population size by the average time at risk
Birth defect rate
prevalence of congenital abnormalities at the time of birth; Cases developed at some point earlier; Denominator doesn't include total population at risk (miscarriages, etc.)
Point prevalence
prevalence of the disease at a certain point in time; "Prevalence" most commonly means point prevalence
Period prevalence
prevalence of the disease over a certain period of time; Includes existing cases at the start of the time period AND new cases that develop during the time period; Need to specify time period (could be a month, a year, 5 years, etc.); Used much less often than point prevalence
Risk
probability that you will get the disease over a certain period of time
Prevalence
proportion; Measures existing cases in the total population; total population diseased; not a rate or a measure of risk
Incidence
proportion; The number or rate of new cases of a particular condition during a specific time
Prevalence Ratio (PR)
ratio of prevalence in the exposed to disease prevalence in the unexposed; PR = (a/ a+b)/ (c/c+d)