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Occurrence of Disease (Prevalence (measure of disease occurrence)…
Occurrence of Disease
Prevalence
(measure of disease occurrence)
expressed as per 1000 persons
(for eg, if constant x 1000)
Factors influencing
prevalence
Case fatality rate
Number of new cases
Duration of disease
Migration of cases
& healthy people
diagnosis
Treatment
Denominator:
no of persons in the pop
at that specified time
measure burden of disease,
help planning health services
Numerator:
no of disease cases present
in pop at a specified time
What proportion of the pop
is affected by the disease at that time?
Definitions
Proportions
fraction - all elements of numerator
incl in denominator;
often x by 100 & expr as %
(dimensionless)
Ratios
fraction - some elements of numerator
are not included in denominator
Count
no of cases
Rates
instantaneous change in one
quantity per unit change in
another quality (usually time)
Measures of morbidity
(=occurrence of disease)
Incidence
no of instances of illness commencing,
during a given period in a
specified pop
new health-related events
in a defined pop
within a specific period of time.
Measured as a freq count,
a rate or a proportion
Incidence rate
Numerator: new events in
specified period
Denominator: av no of persons
exposed to risk
during that period (or pop at risk)
rate at which
NEW
events
occur in a pop
Expressed usually as person time incidence
(the rate per 1000 person-years)
Cumulative incidence,
cumulative incidence rate
no or proportion of a group
(cohort) of people who experience
onset of health-related event
during specified time interval
Usually the same for all members
of a group, but as in lifetime incidence,
it may vary from person to person
wrt age
Attack rate
epidemic/outbreak period:
proportion of group that
experiences outcome
time dimension is uncertain or arbitrary,
technically not a rate
eg exposure = Ekka
outcome = gastro
referred to as attack time for
short period of time
Mortality Rate
Direct Standardisation
uses the death rate in each age group
of our pop of interest against
the std no people in std pop
Assumes we know death rate for each group
expected deaths / people
in a standard pop
std pop- pop everyone uses
(# people in each group)
Indirect Standardisation
info required
age-spec rate in your
study pop
age dist of std pop
Standardised mortality ratio
Use when the age-specific
death rate in your study pop is
not avail - but you do know the
pop profile
observed no death per yr /
expected no deaths per yr
Don't use crude mortality rate-
need to adjust for different
proportions in the age groups
Challenges
Risk of bias
Errors in the numerator
(not representative sample
of general pop)
Use of land lines only
in survey
Truthfulness
Low response rates
people out of scope
Survey instruments
Diagnosis
Errors in the denominator
demography
geography
uncertain of pop size
Data
Many choices of geographical areas
What data is used depends on
What do you want to know about?
Why do you want it
Many sources of data