Intro to Epidemiology Flashcards
What measures disease frequency?
Prevalence (P)
What does prevalence do?
measures existing cases of a disease at a particular point in time or over a period of time.
formula for Prevalence (P)
P = existing cases / entire population (sick or not)
Point Prevalence measures…
… the frequency of disease at a given point in time - “Snapshot in time.”
Period Prevalence measures…
… prevalence over a period of time.
e.g, between 2000 and 2010.
When is incidence similar to prevalence?
When there are no more new cases.
When existing cases die or leave the population at the same rate new cases are developing.
When are incidence and prevalence different?
Increase life expectency - greater number of cases.
Relatively fixed amount of new cases per year.
Incidence Rate
the rate of appearance of new cases over time.
Incidence Rate is a true rate which can assess…
time from exposure to disease.
What are examples of units for incidence rate?
person-years
person-days
Prevalence
the percentage of people in the population with the outcome of interest at any point in time.
Prevalence measures…
… existing cases of a health condition or event.
What type of studies often look at prevalence?
Cross Sectional
What are the two types of prevalence?
Point Prevalence
Period Prevalence
What is prevalence useful for?
Assessing the health status of a population.
Planning health services.
What is prevalence not useful for?
Identifying risk factors - it obscures causal relationships.
Formula for Period Prevalence:
PP= (C+I) / N
C = the # of prevalent cases at the beginning of the time period.
I= the # of incident cases that develop during the period.
N= size of the population for this same period.
Suppose we followed a population of 150 persons for one year, and 25 had a disease of interest at the start of follow-up and another 15 new cases developed during the year.
What is the point prevalence at the start of the period?
25/150 = 0.17 = 17%
Suppose we followed a population of 150 persons for one year, and 25 had a disease of interest at the start of follow-up and another 15 new cases developed during the year.
What is the period prevalence at the start of the period?
(25+15)/150 = 0.27 = 27%
Suppose we followed a population of 150 persons for one year, and 25 had a disease of interest at the start of follow-up and another 15 new cases developed during the year.
What is the point prevalence at the end of the period?
Not known - we don’t know recovery or death.
Suppose we followed a population of 150 persons for one year, and 25 had a disease of interest at the start of follow-up and another 15 new cases developed during the year. None recovered or died.
What is the point prevalence at the end of the period?
=(25+15)/150 = 0.27 or 27%
Suppose we followed a population of 150 persons for one year, and 25 had a disease of interest at the start of follow-up and another 15 new cases developed during the year.
What is the risk for the one year period?
15/125 = 0.12 = 12%
Risk
The probability that an event will occur.
What is measuring risk good for?
Fixed population. Fixed Time.
Rate
A measure of how quickly something happens.
What does rate measure?
Captures changes accounting for dynamic population over varying amounts of time.
Epidemiology
The study of the distribution and determinants of health-related states or events in specified populations and the application of this study to control health problems.
Relative Risk measures…
… association
What is the simple definition for epidemiology?
Epidemiology is the study of health and disease in populations.
What is the unit of study for clinical medicine?
the individual patient
What is the unit of study for epidemiology?
a “population.”
e.g, community, age group, sex, etc.
Clinical Epidemiology
The science of making INDIVIDUAL patient predictions on the basis of POPULATION or group observations.
Endemic
usual presence of a disease within a given population.
A condition regularly found among a group of people.
It requires a steady state of spread.
Epidemic
occurrence of a disease clearly in excess of normal expectancy in a defined community or region.
Pandemic
worldwide epidemic
What are the important questions of distribution?
How common?
Who is affected?
When does it occur?
Where does it occur?
Point Prevalence =
P=C/N
C= # of observed cases at time (t) N= Population size at that time (t)
Descriptive epidemiology deals with…
… the distribution of disease.
Descriptive studies estimate…
…disease frequency and time trends and (sometimes) generate etiologic hypotheses.
Analytic epidemiology deals with…
… determinants of disease.
Analytic Studies
test etiologic hypotheses in order to establish or reject causal links.
What is prevalence affected by?
incidence and disease duration.
crude mortality rate =
crude mortality rate = total deaths / 100,000 population
Attack Rate =
Attack Rate = # new cases / total vulnerable
Case Fatality Rate =
Case Fatality Rate = deaths/cases
Survival Rate =
Survival Rate = number living/# cases
