measures of frequency, association, and impact

cumulative incidence

the proportion of healthy individuals that develop disease over a specified time period, out of those at risk. conveys information on risk of contracting disease

cumulative incidence = number of new cases during a specific time period / population at risk during same period

prevalence definition and equation

proportion of animals in a defined population that has the disease of interest at a specific time

prevalence = number of cases at time t / (DIVIDED BY) number of individuals at time t

prevalence conveys information on

– how widespread or common a disease is

– the chance that an individual has the disease

– your risk of exposure if you contact an individual at random

– the impact on a population, and the costs of control

point prevalence vs period prevalence

point: You find 3 cases of foot rot in a flock of 12 sheep. The prevalence of foot rot in the flock = 3/12 = 0.25 or 25%

This is a “Point prevalence” -single point in time

period: The farmer records 5 cases of foot rot over one month. The period prevalence, based on her observations = 5/12 = 0.416 or 41.6 %.

cumulative relationship between cumulative incidence and point/period prevalence

explain measures of association between exposure and disease/outcome (RR, OR)

relative risk and types of studies relevant for

risk of disease in exposed animals / risk of disease in unexposed animals

cross sectional and cohort

odd ratios, types of study that its relevant for

probability of an event occurring / probability of event Not occurring

case control

interpret measures of population impact such as attributable fraction