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HLTH 240b (10)
Lecture 4

HLTH 240b Lecture 4: Surveillance

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Global Health Studies
HLTH 240b
Marney White

HLTH 240: Epidemiology and Public Health 2/2/17  WHO definition of surveillance: continuous. Systematic collection, analysis and interpretation of health-related data needed for the planning, implementation, evaluation of public health practice 1. Serve as early warning system 2. Document impact of intervention 3. Monitor  CDC outline of tasks for evaluating a surveillance system 1. Describe the public health importance of health event. Three most important categories: a. Total number of cases, incidence, prevalence b. Indices of severity such as the mortality rate and case fatality ration c. Preventability 2. Describe the system to be evaluated a. Vector analysis: Importance is not where disease is reported but potential spread of disease  Measures of disease and morbidity 1. Frequency data 2. Incidence rate a. Number of NEW cases b. Time specific o People observed throughout defined time period o People are NOT observed throughout  Have to make adjustments to account for the fact that people are observed for varying lengths  Person-time  *Pg. 44-45 in textbook c. Population specific (per xx# of people) d. Equation (incidence rate per thousand) = (# NEW cases within time period x 1000)/(# people at risk during that time period) e. Incidence rate/density = # new cases/population time 3. Prevalence a. Overall percentage of people in population with disease o New and old cases b. Equation: = (# cases at specified time)/(# people in population) x 1000 c. Incidence increases prevalence and cures/death decreases prevalence d. Incidence vs prevalence o Incidence considers only new cases  Prevalence considers all, old and new cases o Incidence considers population at risk in denominator  Prevalence considers total population in denominator o If in population someone dies, does not change ? e. # cases depends on: o time interval  Divide by length of time interval  Rate of new cases = (# new cases)/(time interval) o Population size  Divide by population and time  Incidence rate = (# new cases)/(population-time) f. Population-time o Population at risk: people eligible to become a case  Decreases every time new case is added  So with the changing population at risk, person-time changes g. Estimating person-time (3 options) o Add up time that person is at risk  Cumbersome. Must count each individual person. o Add up population at risk during each time segment  Total population time over 6 months divided by 6 months  But, not all cases are at risk for a full month  Improvement: take midpoint at each month for total population time  Assumption: cases develop, on average, in the middle of the month  More precise o Multiply average size of population at risk by the length of time interval  Actual size or approximation of population at risk during 6 months  Population-time approximation  Preferred option 4. Cumulative incidence (incidence proportion) a. 6-month CI = (# new cases during specified per
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