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**in descriptive studies**

Measures in descriptive studies Lecture on measures of disease occurence 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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**Descriptive studies - context in epidemiology**

where who when why hypothesis descriptive analytical population individual observational experimental Clinical trials ecological cross-seccional cohort case-control

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**Descriptive studies - context in epidemiology**

Measuring disease occurrence Descriptive identification of disease determinants Analytical Use: Analysing surveillance data Outbreak investigation Aim: Response / intervention

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**Descriptive studies – measuring disease occurrence**

counts number of cases “we have 5 cases of tetanus” On its own it is very little informative!! Simple / most frequently performed measure in epidemiology Refers to the number of cases of a disease e.g. (or other health phenomenon) Limited usefulness for epidemiologic purposes without knowing size of the source population Who is in the denominator ???? In what time period did they occur???

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**Proportion Ratio Rate Measuring disease occurrence**

What, who is in the denominator ???? In what time period did they occur???

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**Proportion 400 / 1000 40% = 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 males population = 400 / 1000 40%

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

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Rate The division of two numbers TIME INCLUDED in the denominator Speed of occurrence of an event over time 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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**Number of cases of disease**

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 una enfermedad muy frecuente pero de rápida evolución hacia la curación o hacia la muerte puede ser de baja prevalencia, ya que muchos de los casos no los veremos al no coincidir con el momento en que “miremos” la población. una enfermedad de larga duración se irá acumulando en estado de enfermedad y será detectada cuando demos un “corte transversal” a la población. prevalence

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**incidence Measuring disease occurrence (cumulative incidence)**

Number of NEW cases of disease during a period Population at the beginning of the period 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 The cumulative incidence is often also called the incidence proportion In our numerator we will have the number of new cases of disease during a specified period And in the denominator the population at the beginning (AT RISK) of the observation period incidence (cumulative incidence)

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**Number of NEW cases of disease Total person-time of observation**

Measuring disease occurrence Incidence Rate Proportion of the population that acquires or develops a disease in a period of time Speed of developing a disease Number of NEW cases of disease Total person-time of observation It is more precise because it includes the time at risk in the denominator. The incidence rate is the number of new cases divided by the total number of person time observed Denominator: - is a measure of time - the sum of each individual’s time at risk and free from disease

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**Cumulative Incidence = 3 cases / 6 persons = 50% **

Time-person Person 1 l 3 Person 2 l x 4 Person 3 l 6 Person 4 l x 3 Person 5 x l 1 Person 5 l 5 22 p.y 2003 2004 2005 2006 2007 2008 2009 Cumulative Incidence = 3 cases / 6 persons = 50% Incidence Rate = 3 cases / 22 person-years = 0.14 = 14 cases / 100 person-years

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**Outbreak of cholera in country X in March 1999**

Measuring disease occurrence Attack Rate 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! Outbreak of cholera in country X in March 1999 Number of cases Population 18,600 Attack rate % The attack rate is the cumulative incidence during a outbreak, expressed for the entire epidemic period from the beginning to the end of the epidemic It is imprtant to know that it is rather a proportion than a rate What is special about a proportion? Persons in the numerator are always included in the denominator. It indicated the magnitude of a part, related to the total. In epidemiology, it tells us the fraction of the population that is affected. A/A+B The numerical value of a proportion lies from 0 to 1, if multiplied by 100, you get the percentage

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**Measuring disease occurrence developing the disease**

Descriptive Prevalence Incidence Probability of having the disease Probability of developing the disease Risk: Non-technical definition Vague, culture-dependent Unexpected, unusual, dangerous/negative events Risk: Epidemiologic definition Probability that an event will occur Estimated by: observing events among a population during a specified time Burden RISK

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**Measuring disease occurrence**

Let’s talk about ODDS now Probability that an event will happen Probability that an event will not happen Won Lost Total EPIET facilitator team 14 1 15 Odds, is a special measure of disease / event occurence. The odds are often used in horse races and are a ratio The odds are the probability that an event will happen divided the probability that an event will not happen Is a ratio Lets make an example and look at the number of football matches which have been won by facilitators between 1995 and 2004 The odds of facilitators winning the soccer match= the prob that they will win divided probability that they will loose. Facilitaors have won all but one of the 15 matches 14 / 15 Odds of winning = = 14:1 = 14 1 / 15

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**Risks, Odds and 2x2 tables Cases Non cases Exposed a b a+b Non exposed**

c+d a+c b+d 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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**Summary of disease occurence**

PREVALENCE INCIDENCE Cumulative incidence Incidence rate Attack rate ODDS Summarising the measurements of disease occurence We will have the risk Expressed as cumulative incidence or incidence proportion And in outbreak situations the attack rate which is a proportion but not a rate A rate includes a measurement of time in the denominator and is expressed as incidence rate And the odds is a ratio of the probabilty that an event will occur divided by the probability that an event will not occur… it is a measurement of risk for rare events It is limited between 0 and 1…. Meaning between 0 and 100% The attack rate is used in outbreaks but is a proportion and NOT a rate, because the time is not included in the denominator

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**examples Cases of diabetes in 2008 in Spain Spanish population in 2008**

30 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 Proportion Attack rate (proportion) Ratio of time spent in the bar and time spent in the lecture room Mortality rate

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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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Measuring the Occurrence of Disease 1 Sue Lindsay, Ph.D., MSW, MPH Division of Epidemiology and Biostatistics Institute for Public Health San Diego State.

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