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)