Download Tutorial #1 Incidence/Prevalence

Tutorial #1
Incidence/Prevalence
January 17th, 2011
Which to use?
Incidence:
• New cases
Ex.
• Determining etiology (not complicated by
duration)
• P=IxD
Prevalence
• Current cases
Ex.
• Determining the burden of disease
• Planning health services
Incidence
• Cumulative Incidence vs. Incidence Density
Cumulative Incidence
• Can only be calculated when everyone is at risk
for the same amount of time
• Units: proportion, %, per 1000 people
• Commonly used in infectious disease studies
(outbreaks, etc.) and in neonatal studies (short
duration)
Incidence Density
• differential amount of follow-up time
• Either people enter the study at different
times
• People leave the study at different times
• or both
• Units: Person-Time
Person-Time
• A way of accounting for differential amount of follow-up
time
• Takes into account the amount of time each person
contributes to the follow-up period
Allows us to account for
• Loss-to-follow-up (death, drop-out)
• Different entry points into the study
Person Time
• Example: If I want to know my risk of acquiring a
nosocomial infection if I go to Hospital X
• The numerator: Total number of nosocomial
infections at Hospital X
• The denominator: Number of people at risk?
• Period of time?
• Patients are at risk for different amounts of time
• How many people acquire an infection out of the
total amount of time patients are at risk?
Numerator= # cases
Denominator = # patients * the amount of time
each patient was in the hospital
= total time patients spent at risk
- Accounts for patients spending more time at the
hospital than others (higher risk if more time
spent at risk)