Lecture 2 Flashcards
prevalence formula
# people who have disease at given point in time \_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_ # of people in population of interest
cumulative incidence formula
# new cases of disease in a given time period \_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_ # people who can develop that disease
prevalence in relation to incidence and duration
prevalence = indidence x average duration
prevalence
- the probability of having a disease at at given point in time
- the pre-test probability
- how common a disease is
- gives you baseline probability
incidence
- the proportion of new cases of disease in the population at risk of getting the disease over a given period of time
- also known as risk
- tells you about what could happen in the future
incidence versus prevalence
- incidence based on new cases
- prevalence based on new and old
- incidence based on cases that develop over a specified period of time
- prevalence is based on cases that exist at a single point in time
incidence density
- the number of new cases of disease that develop over a given time period divided by person-time at risk of disease
if incidence increases and average duration remains the same
- prevalence increases
if average duration of a disease increases and incidence stays the same
- prevalence increases
attack rate
- the proportion of people who develop the disease, among those at risk, during a specified time period
attack rate formula
new cases of disease in a given time period
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persons at risk
case fatality rate
- the proportion of people affected with a disease who die from it, in a given time period
case fatality rate formula
deaths from a disease in a given time period
____________________________________
number of persons who have that disease
mortality rate
- the proportion of people who die, among those at risk of death, in a given time period
mortality rate formula
deaths in a given time period
__________________________
population at risk of dying
how to get estimates of pre-test probability?
- knowledge of prevalence in your clinical site
- medical literature
- clinical predictions
- clinical experience
test threshold
- separates “don’t test, don’t treat” zone from “test” zone
treat threshold
- separates “test” zone from “don’t test, treat” zone
determines where the thresholds are set
- accuracy of test
- risk of test
- seriousness of illness and benefit of treatment
- risk of treatment
accuracy of test
- test threshold will be lower for more accurate tests
risk of test
- test threshold will be higher for more risker; invasive tests
seriousness of illness and benefit of treatment
- test and treatment thresholds will be lower for serious illnesses with beneficial treatment
risk of treatment
- treatment threshold will be higher for riskier treatment
if pre-test probability is below test threshold
- not wise to test for the condition at all because a positive test is more likely to be a false positive than a true positive
rule in
- moved above treat threshold
rule out
- moved below test threshold
if pre-test probability is above the treat threshold
- don’t test, just treat
pattern recognition
- one of the most efficient diagnostic strategies used by seasoned clinicians
point prevalence
- measured at a single point in time for each person
period prevalence
- describes cases that were present at any tine during a specified period of time
another name for prevalence studies
- cross-sectional studies
- people are studied at a “cross-section” of time
cohort
- population under examination in an incidence study
- group of people having something in common when they are first assembled and are then followed over time for the development of outcome events
another name for incidence studies
- cohort studies
cumulative incidence
- describe the rate of new events in a group of people of fixed size, all members of which are observed over
a period of time. - new cases are accumulated over time
incidence density
- to measure the number of new cases emerging in an ever-changing population, one in which individuals are under study and susceptible for varying lengths of time.
- common for clinical trial companies
denominator of incidence density measure
- person-time
dynamic populations
- those in which some individuals in the population are entering and others leaving as time passes
