Epidemiology (II) Rates Flashcards
Define incidence.
New rates of disease among the at-risk population
Define prevalence.
Number of sick individuals in the population
Define cumulative incidence.
of new cases of disease in a specified population at risk during a specified time period / at-risk population
How is cumulative incidence often expressed?
As new cases per _____ at-risk individuals
OR
a percentage
What do incidence rates allow us to infer that prevalence rates do not?
Risk of contracting disease
What assumption must be met for cumulative disease rates to be most useful?
There must not be any attrition in the population follow-up
What is incidence density (incidence rate)?
An individual’s risk per unit of person-time (e.g. person-month, person-year, etc.) (expressed as a number ‘per 100,000 person-years’)
Numerator = incidence of disease among at-risk individuals
Denominator = the sum of the units of time that each individual was at risk and was observed.
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Why would an individual no longer be tracked during a study of cumulative incidence?
If the individual is no longer at-risk (they die, stop being at risk, or are lost to follow-up)
What is period prevalence (as opposed to point prevalence)?
Prevalence during a specified time period
(basically starting prevalence + incidence during that period)
Cumulative incidence is directly interpretable as what?
Risk
What are incidence rates interpretable as?
Indirect estimates of risk
What is the relationship between incidence rate and cumulative incidence?
Cumulative incidence = incidence rate * time period
(assuming no secular trend in new disease incidence)
What is the relationship between the incidence rate and prevalence?
Prevalence = incidence * duration of disease
(assuming low disease incidence and stable incidence over time)
What are the steps for identifying a new case of disease?
Step 1: determine who has the disease and who does not.
Step 2: follow-up with those who did not have the disease at baseline (i.e. ignore baseline prevalent cases)
Define mortality rate (basically an incidence rate, but with deaths instead of new cases).
(# of deaths in a specified population during a specified time period) / (Person-time contributed in the specified population during the specified time period)