Frequency of disease Flashcards
why should we care about disease frequency
to allow us to understand the size of the problem
spot outbreaks of a disease
assess risks to and management of a condition
aids planning of resources
case definition
standardised method of classifiction
exact definition depends on source of data and use
population at risk
people who would be included among the cases if they developed the disease
person time at risk
what does population at risk usually involve
place
time
demographics
defining event
what are the two types of population
fixed
dynamic
fixed population
membership is permanent
attended a specific event
born a certain time
dynamic population
membership is based current status
currently studying at uni
lives in specific city
most basic epidemiological measure
count
count
expressed as integers
answers the question “how many people have this disease?”
often the numerator of many measures
important to distinguish between incident and prevalent cases
limitations of count
doesn’t provide much information/context
pre-cursor to calculation
calculating a ratio
divide one number by the other
ranges from zero to infinity
x and y may be related or completely independent
calculating proportion
ratio where the numerator x divided by x +y
ranges from 0 to 1
often expressed as a percentage
calculating rate
expressed as x/T
T= component of time
ranges zero to infinity
measures speed at which things occur
prevalence
proportion
not a rate as no time component involved
measures proportion of existing disease in the population at a given time
dimensionless, positive number 0 to 1
point prevalence
proportion of a population who have a disease at any one time
point prevalence equation
number of cases of disease divided by the total population (cases and non-cases) at a particular point in time
period prevalence
proportion of a population who have. disease during a specified time period
period prevalence equation
number of cases of disease divided by the total population (cases and non-cases) during a period of time
what is the estimation of prevalence useful for
determination of sickness load
planning health services
examining health behaviours
factors affecting prevalence
disease duration
case fatality
changes in treatment
incidence
ascertainment of cases
migration
long duration
7 months
short duration
3 months
what are the key measures of frequency
prevalence
incidence
incidence
measures the occurrence of new cases in a population at risk over tine
can be measures as a proportion or a rate
most fundamental epidemiological indicator:
- measures force of morbidity
- measures conversion of health status (proportion/rate)
measuring conversion of health status
proportion divided by rate
incidence proportion
proportion of people who develop a disease during a specific period of time
dimensionless, positive number 0 to 1
incidence proportion (cumulative incidence) equation
number of new cases of disease during a specified time period divided by total population at risk during that time period
incidence proportion (cumulative incidence)
appropriate for fixed populations and short follow-up
must specific time period of observation because risk changes with time
not appropriate for long-term follow-up due to potential loss of subjects
assuming: complete follow-up, same risk over time
cumulative incidence equation
number of new cases over a period divided by the number of population at risk over the same period
estimation of incidence proportion is useful for
surveillance of a disease
understanding the aetiology
planning service use
incidence rate
measures the speed that new cases develop during specified time period
cases per person-time
follow-up may be incomplete
risk period not the same for all subjects
incidence rate equation
number of new cases divided by the person-time at risk during the study period
person-time
accounts for all the time each person is in the population at risk
length of time for each person is the person-time
sum of person-times is called the total person-time at risk for the population
investigator assigned exposures
with a randomly allocated experimental study
randomised controlled trial
investigator assigned exposures
with a non randomly allocated experimental study
non-randomised controlled trial
investigator didn’t assign exposures
with a comparison group in the observational study
with an exposure to outcome direction of analytical study
cohort study
investigator didn’t assign exposures
with a comparison group in the observational study
with an outcome to exposure direction of analytical study
case-control study
investigator didn’t assign exposures
with a comparison group in the observational study
with an exposure and outcome at the same time direction of analytical study
cross-sectional (analytical) study
investigator didn’t assign exposures
without a comparison group in the observational study
with descriptive study
cross-sectional (prevalence) study
ecologic study
case report/time
what are cross-sectional (prevalence) surveys
observational, descriptive
survey of health in a ample of a well-defined population
key elements: identification of target population, procedure for sampling, method of data collection, definitions used
analysis: point or period prevalence estimate
strengths of cross-sectional prevalence surveys
quick and inexpensive
potential to include large numbers
useful to estimate size of the problem
can describe the variation by person place and time
limitation of cross-sectional prevalence survey
prevalence estimates suseceptibility to non-response bias
tendency to selectively include problems with long duration
strengths of cohort studies
can measure incidence
investigate natural history and survival
investigate multiple outcomes
minimise recall bias
accurate assessment of time-varying exposures
easier to establish temporal sequence of exposure and outcome than case-control
limitations of cohort studies
expensive and time consuming
can be limited to available information on prior exposures
inefficient for rare outcomes
potential selection bias
what are cohort studies
observational and analytical
identify groups of individuals and measure exposure status
starts with exposures and looks forward to possible effects
influence of time
period effect
cohort effect
changes in prevalence
time of measurement
change in case definition
introduction or impact off an intervention or treatment
changes in duration or incidence of a disease
dynamic population
changes in incidence
new risk factor
changes in virulence
migration
changes in intervention strategy
is the variation real
may be due to:
chance
error
data collection/coding
analysis
incidence VS prevalence
description of new cases of disease in a particular time period
rate or risk of developing a disease in a specified time period
requires the timing of onset of disease to be known
monitoring of disease
prevalence VS incidence
snapshot of disease in a particular population at a point in time
proportion of the population within the disease
point prevalence- only suitable for disorders that persist for a reasonable Tim
survey
