Lecture 2, part 3 Flashcards
Formula for prevalence
Total # of cases of dz (new + old) / Total # of persons in the pop at that specified time
Aspects of point prevalence
Numerator includes all existing cases at a specific point in time
Generally more common
Aspects of period prevalence
Numerator contains all existing cases plus all new cases occurring during the period of observation
Denominator is total pop, including all cases
-Mid-year pop often used
Formula for incidence
of new cases of dz that occur during a specified period of time/# of persons who are at risk of developing the dz during that period of time
What is incidence a measure of?
New events
Risk
How is incidence a transition?
From non-diseased to diseased state
Any person in the _______ must have the potential to become part of the group contained in the ________.
Denominator
Numerator
When are ppl not at risk in terms of incidence?
Once they have the dz, die, or leave the study
Incidence rate formula
of new cases during a specific period/total person time at-risk during that time
Definition of incidence rate
The speed at which new cases are occurring in a pop at risk
What is the denominator of the incidence rate?
Person time, the sum of time each person is at-risk but free of dz
Aspects of person-time
Person-time units are assumed to be equivalent
Conceptually simple, but operationally difficult to measure
What is another term for incidence rate?
Incidence density
Formula for cumulative incidence
of new cases during specified period/# of at-risk individuals free of dz at beginning of time period
Cumulative incidence definition
Measure of the proportion of ppl, during a specific time, who move from a healthy state to a dz state
Characteristics of cumulative incidence
Assumes fixed pop (everyone followed for entire period)
Time referent is still critical in the estimate, but actual person-time is ignored
What is another term for cumulative incidence?
Incidence proportion
Formula of relationship between cumulative incidence and incidence rate
CI = IRi x ti
Where CI is cumulative incidence
IR is incidence rate over a specified period of time
and
t is the specified period of time
Assumptions behind the cumulative incidence in relation to incidence rate formula
No competing risks (denominator assumes everyone would have survived dz-free or survived to the end of the interval)
Data reflect a longitudinal experience
Incidence rate is constant with time
-When incidence rate is not constant: CI = Σ(IRi x ti)
_______ can be portrayed as a percentage, but __________ cannot
Cumulative incidence
Incidence rates
What type of number is prevalence?
Proportion
What units are in prevalence?
None
What is the range of prevalence?
0 to 1
What is the numerator of prevalence?
Existing cases
