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Lecture 02b.pptx

# Lecture 02b.pptx - Lecture 2 Part II Measures of disease...

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Lecture 2 – Part II Measures of disease occurrence Ratios, proportions, rates Risk Incidence rates Prevalence

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What is the pre-requisite 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

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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 person-years)

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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 births -Proportions can also be expressed as a percentage: i.e. percentage of fetal deaths=0.6%

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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 1980-84 1 per year 1996-99 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

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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

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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 There are two specific types of incidence measures: Cumulative Incidence (CI) Person-Time Incidence Rate
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