POPM 3240 - Measures of Frequency/Screening+Diagnostics/Communication
+ Trust ACTUAL UPDATED QUESTIONS AND CORRECT ANSWERS
why do we measure disease frequency? to describe:
-estimate burden of disease for prioritization
-determine disease current status- establish "normal" within a population to
interpret changes over time
to compare:
-between populations
-over time (ex. flu seasons)
-between risk factor groups (ex. age- elderly vs young)
-evaluate impact of preventative measures
three measures of frequency counts, proportions, rates
proportions -number with disease divided by the populations size (ex. A/A+B)
-values range from 0 to 1, often expressed as a percentage
rates an expression of an event occurrence in a defined population in a specified
period of time (how fast do new cases occur in a populations within a specific
timeframe)
prevalence the number of existing* cases of disease in the population (at a certain point or
period); a proportion; doesn't tell us when disease occured (ex. yesterday or last
year); doesn't tell us about the probability of getting the disease (risk)
-prevalence is proportional to incidence x duration
incidence the number of new cases of disease in the populations (during a given time
period); can be expressed as risk (a proportion) or rate
, point prevalence proportion of the population that has the disease of interest at a single point in
time
= # individuals with disease* at time t/# individuals in a population at time t
period prevalence proportion of the population that has the disease of interest during a certain
period
= # individuals with disease* during time interval t/# individuals in a population
during time interval t
withdrawal from a population can occur due to the end of the study, individual left the cohort, was lost to
follow-up, died from a cause other than the one of interest
to measure incidence we need... two measurements: number of disease-free at start of period, number that
develop disease during period of observation (want to know if they are in fact
new cases)
numerator: # of new cases or events
denominator: population at risk of developing the outcome
incidence risk proportion of unaffected individuals who on average will develop the disease of
interest over a specified time period (likelihood/chance of developing disease in
that time); only of interest to us if they get the disease in that period- because we
are looking at disease risk
incidence rate measures the rapidity or average speed with which newly diagnosed cases of the
disease develop in the population; denominator is the sum of units of time each
individual was at risk and observed: can either be exact (preferred method) or
approximate
incidence rate measures two things... # new cases / # at risk in a population
# new cases / time period
therefore combined: # new cases / (# at risk in a population x time period)
measured in units of individual-time
individual-time accounts for how long and how many people were at risk in a population (how
much time-at-risk did each individual in the population contribute); interpretation
at a population level: 25 person-years means that the study followed some
combo of people and time that added up to 25 years of observation (5 ppl at risk
for 5 years; 25 ppl at risk for 1 year; 1 person at risk for 25 years)
interpreting incidence rate to interpret at the population level you can multiply the numerator and
denominator by a factor of 10 to make it more intuitive
ex. 0.09 cases per person-year multiplied by 100 means if we followed 100 people
for 1 year we would expect to see 9 new cases of the disease
prevalence and incidence relationship prevalence = incidence x duration
-general relationship in a steady state situation (i.e. incidence and prevalence will
generally not change, and in-migration = out-migration)
-so prevalence>incidence for chronic disease and vice versa for acute
morbidity sickness; crude (overall) or cause-specific; always measured as incidence (risk or
rate)
+ Trust ACTUAL UPDATED QUESTIONS AND CORRECT ANSWERS
why do we measure disease frequency? to describe:
-estimate burden of disease for prioritization
-determine disease current status- establish "normal" within a population to
interpret changes over time
to compare:
-between populations
-over time (ex. flu seasons)
-between risk factor groups (ex. age- elderly vs young)
-evaluate impact of preventative measures
three measures of frequency counts, proportions, rates
proportions -number with disease divided by the populations size (ex. A/A+B)
-values range from 0 to 1, often expressed as a percentage
rates an expression of an event occurrence in a defined population in a specified
period of time (how fast do new cases occur in a populations within a specific
timeframe)
prevalence the number of existing* cases of disease in the population (at a certain point or
period); a proportion; doesn't tell us when disease occured (ex. yesterday or last
year); doesn't tell us about the probability of getting the disease (risk)
-prevalence is proportional to incidence x duration
incidence the number of new cases of disease in the populations (during a given time
period); can be expressed as risk (a proportion) or rate
, point prevalence proportion of the population that has the disease of interest at a single point in
time
= # individuals with disease* at time t/# individuals in a population at time t
period prevalence proportion of the population that has the disease of interest during a certain
period
= # individuals with disease* during time interval t/# individuals in a population
during time interval t
withdrawal from a population can occur due to the end of the study, individual left the cohort, was lost to
follow-up, died from a cause other than the one of interest
to measure incidence we need... two measurements: number of disease-free at start of period, number that
develop disease during period of observation (want to know if they are in fact
new cases)
numerator: # of new cases or events
denominator: population at risk of developing the outcome
incidence risk proportion of unaffected individuals who on average will develop the disease of
interest over a specified time period (likelihood/chance of developing disease in
that time); only of interest to us if they get the disease in that period- because we
are looking at disease risk
incidence rate measures the rapidity or average speed with which newly diagnosed cases of the
disease develop in the population; denominator is the sum of units of time each
individual was at risk and observed: can either be exact (preferred method) or
approximate
incidence rate measures two things... # new cases / # at risk in a population
# new cases / time period
therefore combined: # new cases / (# at risk in a population x time period)
measured in units of individual-time
individual-time accounts for how long and how many people were at risk in a population (how
much time-at-risk did each individual in the population contribute); interpretation
at a population level: 25 person-years means that the study followed some
combo of people and time that added up to 25 years of observation (5 ppl at risk
for 5 years; 25 ppl at risk for 1 year; 1 person at risk for 25 years)
interpreting incidence rate to interpret at the population level you can multiply the numerator and
denominator by a factor of 10 to make it more intuitive
ex. 0.09 cases per person-year multiplied by 100 means if we followed 100 people
for 1 year we would expect to see 9 new cases of the disease
prevalence and incidence relationship prevalence = incidence x duration
-general relationship in a steady state situation (i.e. incidence and prevalence will
generally not change, and in-migration = out-migration)
-so prevalence>incidence for chronic disease and vice versa for acute
morbidity sickness; crude (overall) or cause-specific; always measured as incidence (risk or
rate)
