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Download Tutorial #1 Incidence/Prevalence
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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)