Measure of Disease Occurrence Flashcards
why measure disease?
know what is going on in a population and how disease is changing it
compare disease in groups
T/F “risk factor” is another word for “determinant” of disease
true
epidemiologist are concerned with
presence of existing health problems in a population
occurrence of new health events in a population
and their measurement (quantification)
what do epidemiologists measure
amount of disease in a population at one point in time or over a period of time
change in amount of disease in a population over a period of time
stratified=
population subdivided by group (breed, age, sex etc)
what should be defined when measuring disease occurrence
what is being measured
study population
place of location of study population
time period of study
study population
subjects of the study
usually a sample from the “source” population
source population
population from which subjects were drawn
target population
population to which we may want to generalize our results
most common types of measurement used
counts
proportions
ratios
rates
counts
number of animals that have a disease
convey little information on their own
proportions
count of animals with the disease as a fraction of the total animals that could be diseased
diseased animals/ total in study
what is the most commonly used proportion in epidemiology
prevalence
ratios
fraction which the numerator is not part of the denominator
measure of frequency with which an event occurs in a defined population over a specified period of time
expression of the change in the amount of disease in a defined population per unit of time
what is a commonly used ratio in epidemiology
‘odds’ ratio
2 types of prevalence
point
period
2 types of incidence
cumulative incidence
incidence rate
what is prevalence?
proportion of the study population that is diseased at any one time
amount of disease in the population at any one time
T/F prevalence provides information about how frequently you might expect to see the condition in your practice
true
point prevalence
proportion of the study population that is diseased at a single point in time
what is the most common measurement of prevalence
point prevalence
how is point prevalence determined
cross-sectional studies
how do you calculate point prevalence
number of cases of disease in the population at a particular time
——————————————–total population
what can point prevalence be expressed as
percent
proportion
fraction
what is period prevalence
proportion of the study population that is diseased during a specified period of time
T/F period prevalence includes old and new cases
true
doesnt tell us about when the animals became diseased
T/F period prevalence is the probability that an animal in the study population is diseased during the period of the study
true
how do you calculate period prevalence
total population over that period of time
what is incidence
number of new cases of disease that occur in the study population over time
T/F incidence tells us how frequently non-diseases/susceptible individuals become diseased over time
true
incident cases=
new cases of disease
incidence is interpreted as…
probability of a disease-free animal from the study population becoming diseased
risk of becoming diseased
how do you calculate cumulative incidence
total population at risk
What is cumulative incidence?
proportion of disease-free (susceptible) individuals in the study population who became diseased during a specified period of time
how is cumulative incidence interpreted
probability (risk) of a susceptible individual in the study population becoming diseased during the study period
how is cumulative incidence expressed
a number
dimensionless
what are the assumptions with cumulative incidence
all individuals in the study pop are at risk of getting the disease of interest
every individual in the study pop is assessed at the start of the study
entire population must be followed from the start of the study until the end
attack rate
cumulative incidence during an outbreak (specific type of cumulative incidence)
applied to a narrowly-defined population
how is attack rate interpreted
probability (risk) of becoming diseased during the course of an outbreak
how do you calculate attack rates
total population at risk (exposed)
how is cumulative incidence can be expressed
number
dimensionless fraction
may not have a reference to time as it is assumed to be the duration of the outbreak
what is incidence rate
instantaneous rate of occurrence of new cases of disease among non-diseased animals in the population
when is incidence rate used
when animals are entering and leaving the population
T/F incidence rate can be easily interpreted at the individual animal level
false
how do you calculate incidence rate
sum of the length of time
incidence rate is expressed as
number of cases per animal-time at risk
T/F ONLY the incidence rate is reported as animal-time at risk
true
period prevalence and cumulative incidence are reported as cases/animals/time
prevalence vs incidence
prevalence: measure of amount of disease in a population
incidence: measure of the rate of disease occurrence
relationship between prevalence and incidence
a new case (incidence case) becomes and old case (prevalent case) and remains so until recovery or death
T/F the prevalence of a disease may increase when incidence remains stable but survival of cases improves
true because reasons
which is used for chronic conditions
prevalence
used to assess causes for disease
incidence
best measure to assess increasing or decreasing trends in disease frequency
incidence
disease risk
probability of becoming diseased
at risk
means individual is not diseased an is capable of becoming diseased
risk factors
factors that increase the probability of becoming diseased
exposure to a risk factor
individual has come into contact with a risk factor or has the risk factor (while not diseased)
mortality rate
incidence of death in a given time period
how to calculate mortality rate
total population
case-fatality rate
number of cases of a specific disease that are fatal, within a specific time following disease onset or diagnosis
how to calculate case-fatality rate
total number of individuals with the disease (cases)
