Presentation on theme: "in descriptive studies"— Presentation transcript:
1 in descriptive studies Measuresin descriptive studiesLecture on measures of disease occurenceAlicia Barrasa
2 Descriptive studies - context in epidemiology Epidemiology studies the occurrence of disease:the frequency and distribution of diseases and their determinants in the populationfrequency and distribution: who, when, wheredeterminants: why
3 Descriptive studies - context in epidemiology wherewhowhenwhyhypothesisdescriptiveanalyticalpopulationindividualobservationalexperimentalClinical trialsecologicalcross-seccionalcohortcase-control
4 Descriptive studies - context in epidemiology Measuring disease occurrenceDescriptiveidentification of disease determinantsAnalyticalUse:Analysing surveillance dataOutbreak investigationAim:Response / intervention
5 Descriptive studies – measuring disease occurrence countsnumber of cases“we have 5 cases of tetanus”On its own it is very little informative!!Simple / most frequently performed measure in epidemiologyRefers 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 populationWho is in the denominator ????In what time period did they occur???
6 Proportion Ratio Rate Measuring disease occurrence What, who is in the denominator ????In what time period did they occur???
7 Proportion 400 / 1000 40% = The division of 2 numbers Numerator INCLUDED in the denominatorIn general, quantities are of same natureIn general, ranges between 0 and 1Percentage = proportion x 100malespopulation=400 / 100040%
8 RatioThe division of two numbersNumerator NOT INCLUDED in the denominatorAllows to compare quantities of different naturemalesfemales=5 / 22.5 /1bedsdoctors850 / 10=85 / 13 / 1=participantsfacilitators
9 RateThe division of two numbersTIME INCLUDED in the denominatorSpeed of occurrence of an event over timeBirths in 2007Population in 2007=2000 /= = 1.3 per 10,000 inhabitants per yearRate may be expressed in any power of 10:100, 1000, 10000, …
10 Number of cases of disease Measuring disease occurrenceNumber of cases of diseasePopulationNumber of cases of a disease in a given population at a specific timeProportion of the population that had the disease at a given timeProbability of having the diseaseuna 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
11 incidence Measuring disease occurrence (cumulative incidence) Number of NEW cases of disease during a periodPopulation at the beginning of the periodNumber of new cases of a disease in a given population at a specific timeProportion of the population that acquires or develops a disease in a period of timeProbability of developing a diseaseThe cumulative incidence is often also called the incidence proportionIn our numerator we will have the number of new cases of disease during a specified periodAnd in the denominator the population at the beginning (AT RISK) of the observation periodincidence(cumulative incidence)
12 Number of NEW cases of disease Total person-time of observation Measuring disease occurrenceIncidence RateProportion of the population that acquires or develops a disease in a period of timeSpeed of developing a diseaseNumber of NEW cases of diseaseTotal person-time of observationIt 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 observedDenominator:- is a measure of time- the sum of each individual’s time at risk and free from disease
14 Outbreak of cholera in country X in March 1999 Measuring disease occurrenceAttack RateCumulative incidence during an outbreakExpressed for the entire epidemic period, from thefirst to the last caseNot really a rate but a proportion!Outbreak of cholera in country X in March 1999Number of casesPopulation 18,600Attack 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 epidemicIt is imprtant to know that it is rather a proportion than a rateWhat 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+BThe numerical value of a proportion lies from 0 to 1, if multiplied by 100, you get the percentage
15 Measuring disease occurrence developing the disease DescriptivePrevalenceIncidenceProbability ofhaving the diseaseProbability ofdeveloping the diseaseRisk: Non-technical definitionVague, culture-dependentUnexpected, unusual, dangerous/negative eventsRisk: Epidemiologic definitionProbability that an event will occurEstimated by:observing events among a populationduring a specified timeBurdenRISK
16 Measuring disease occurrence Let’s talk about ODDS nowProbability that an event will happenProbability that an event will not happenWonLostTotalEPIET facilitator team14115Odds, is a special measure of disease / event occurence.The odds are often used in horse races and are a ratioThe odds are the probability that an event will happen divided the probability that an event will not happenIs a ratioLets make an example and look at the number of football matches which have been won by facilitators between 1995 and 2004The 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 matches14 / 15Odds of winning = = 14:1 = 141 / 15
17 Risks, Odds and 2x2 tables Cases Non cases Exposed a b a+b Non exposed c+da+cb+dRisk 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/cOdds of exposure among non cases = (b/(b+d))/d/(b+d))= b/d
18 Summary of disease occurence PREVALENCEINCIDENCECumulative incidenceIncidence rateAttack rateODDSSummarising the measurements of disease occurenceWe will have the riskExpressed as cumulative incidence or incidence proportionAnd in outbreak situations the attack rate which is a proportion but not a rateA rate includes a measurement of time in the denominator and is expressed as incidence rateAnd 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 eventsIt 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
19 examples Cases of diabetes in 2008 in Spain Spanish population in 2008 30 guests with diarrhoea300 wedding guests80 hours at the Lazareto bar150 hours in the lecture roomNumber of deaths in 2007 in SpainSpanish population in 2007ProportionAttack rate (proportion)Ratio of time spent in the bar and time spent in the lecture roomMortality rate
20 References Epidemiology, an introduction. Rothman KJ. Epidemiology in Medicine. Henneckens CH, Buring JE.Modern infectious disease epidemiology. Giesecke J.Dictionary of epidemiology. Last J.