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Measures in descriptive studies Alicia Barrasa

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Descriptive studies - context in epidemiology Epidemiology studies the occurrence of disease: the frequency and distribution of diseases and their determinants in the population frequency and distribution: who, when, where determinants: why

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Clinical trials case-control cohort ecological populationobservational experimental descriptiveanalytical cross- seccional individual hypothesis Descriptive studies - context in epidemiology who where when why

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Measuring disease occurrence identification of disease determinants Descriptive Analytical Use: Analysing surveillance data Outbreak investigation Aim: Response / intervention Descriptive studies - context in epidemiology

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Who is in the denominator ???? In what time period did they occur??? number of cases we have 5 cases of tetanus On its own it is very little informative!! counts Descriptive studies – measuring disease occurrence

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Proportion Ratio Rate What, who is in the denominator ???? In what time period did they occur??? Measuring disease occurrence

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The division of 2 numbers Numerator INCLUDED in the denominator In general, quantities are of same nature In general, ranges between 0 and 1 Percentage = proportion x 100 Proportion males population = 400 / 1000 = 40%

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The division of two numbers Numerator NOT INCLUDED in the denominator Allows to compare quantities of different nature Ratio beds doctors 850 / 10 == 85 / 1 males females = 5 / 2 = 2.5 /1 3 / 1 = participants facilitators

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The division of two numbers TIME INCLUDED in the denominator Speed of occurrence of an event over time Rate Births in 2007 Population in 2007 = 2000 / = = = 1.3 per 10,000 inhabitants per year Rate may be expressed in any power of 10: 100, 1000, 10000, …

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Measuring disease occurrence Number of cases of disease Population –Number of cases of a disease in a given population at a specific time –Proportion of the population that had the disease at a given time –Probability of having the disease prevalence

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Number of NEW cases of disease during a period Population at the beginning of the period Measuring disease occurrence –Number of new cases of a disease in a given population at a specific time –Proportion of the population that acquires or develops a disease in a period of time –Probability of developing a disease incidence (cumulative incidence)

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Incidence Rate Number of NEW cases of disease Total person-time of observation Proportion of the population that acquires or develops a disease in a period of time Speed of developing a disease Denominator: - is a measure of time - the sum of each individuals time at risk and free from disease Measuring disease occurrence

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Cumulative Incidence = 3 cases / 6 persons = 50% Incidence Rate = 3 cases / 22 person-years = 0.14 = 14 cases / 100 person-years l x x l l x l l l Person p.y Person 2 Person 3 Person 4 Person 5 Time-person

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Cumulative incidence during an outbreak Expressed for the entire epidemic period, from the first to the last case Not really a rate but a proportion! Attack Rate Measuring disease occurrence Outbreak of cholera in country X in March 1999 Number of cases 490 Population 18,600 Attack rate 2.6%

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Measuring disease occurrence Descriptive PrevalenceIncidence Probability of having the disease Probability of developing the disease RISKBurden

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Measuring disease occurrence Lets talk about ODDS now Probability that an event will happen Probability that an event will not happen WonLostTotal EPIET facilitator team / 15 Odds of winning = = 14:1 = 14 1 / 15

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CasesNon cases Exposedaba+b Non exposedcdc+d a+cb+d Risks, Odds and 2x2 tables Risk of being a case in exposed = a / (a+b) Risk of being a case in non exposed = c / (c+d) Odds of exposure among cases = (a/(a+c))/(c/(a+c))= a/c Odds of exposure among non cases = (b/(b+d))/d/(b+d))= b/d

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PREVALENCE INCIDENCE –Cumulative incidence –Incidence rate –Attack rate ODDS Summary of disease occurence

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examples Cases of diabetes in 2008 in Spain Spanish population in guests with diarrhoea 300 wedding guests 80 hours at the Lazareto bar 150 hours in the lecture room Number of deaths in 2007 in Spain Spanish population in 2007

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References Epidemiology, an introduction. Rothman KJ. Epidemiology in Medicine. Henneckens CH, Buring JE. Modern infectious disease epidemiology. Giesecke J. Dictionary of epidemiology. Last J.

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