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Unformatted text preview: Lecture 2 – Part II Measures of disease occurrence Ratios, proportions, rates Risk Incidence rates Prevalence What is the prerequisite for conducting epidemiologic investigations? • Ability to quantify the occurrence of disease (outcome) • What is the most basic measure of disease occurrence? City A has 100 hospitalizations for asthma and City B has 250 hospitalizations for asthma 1. Would you conclude that severe asthma is more common in City A or City B? 2. What do you need to account for to compare the occurrence of asthma in these two populations? Example How can we quantify the occurrence of disease? • Rates of occurrence allow us to quantify and compare burden of disease New cases of Hepatitis City A 58 City B 35 Requires knowledge of… Annual rate of occurrence of Hepatitis A • City A: 58 per 25,000 per year • City B: 35 per 14 000 for per year Apply Rate base: per 100, 1000, 10000…to achieve a whole number in the numerator Annual rates of hepatitis: City A = (58 /25,000) x 100,000 =232 cases* City B= (35 /14000) x 100,000 =250 cases* *(per 100,000 personyears) Ratios, Proportions, Rates Ratio: x/y no relationship implied between numerator (x) and denominator (y) Example: sex ratios of stomach cancer 1960: 1.3 : 1 2006: 2.5 : 1 (male:female) Examples of mathematical parameters used to relate number of cases to size of source population Ratios, Proportions, Rates Proportion: x/y Those who are in the numerator (x) are also included in the denominator (y) Example: proportion of fetal deaths 6.0 fetal deaths/1000 total number of birthsProportions can also be expressed as a percentage: i.e. percentage of fetal deaths=0.6% Ratios, Proportions, Rates Hypothetical Data from Queen’s University Issue: Anorexia Nervosa among university students Time Period Duration Students Cases early 1980’s 4 yrs 14,000 4 late 1990’s 3 yrs 17,000 4 Rate: x per y relationship between x and y  y includes an element of time Ratios, Proportions, Rates Hypothetical Data from Queen’s University Issue: Anorexia among university students Common time period 198084 1 per year 199699 1.3 per year Common population size (denominator) 1980’s 1 per 14,000 per year = 0.7 per 10,000 per year = 7/10 5 /yr 1990’s 1.3 per 17,000 per year = 0.76 per 10,000 per year = 7.6/10 5 /yr Risk Perception, Incidence and Prevalence Risk Perception • People have very different attitudes toward risk. • There are two components of risk perception Probability that an adverse outcome will result from exposure (or activity). This is an epidemiological measurement Complex mixture of social and psychological factors Incidence • Deals with what is new • Number of new events or cases of disease that develop in a population of individuals at risk during a specified time interval •...
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This note was uploaded on 02/23/2012 for the course EPID 301 taught by Professor Richardson&aronson during the Spring '09 term at Queens University.
 Spring '09
 Richardson&Aronson

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