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  1. 1. EPIDEMIOLOGYwww.drjayeshpatidar.blogspot.comwww.drjayeshpatidar.blogspot.in
  2. 2. • Epidemiology, "the study of what is uponthe people", is derived from the Greekterms epi = upon, among; demos =people, district; logos = study, word,discourse; suggesting that it applies onlyto human populations. Butwww.drjayeshpatidar.blogspot.in
  3. 3. History• The Greek physician Hippocrates issometimes said to be the uncle ofepidemiology. He is the first person knownto have examined the relationshipsbetween the occurrence of disease andenvironmental influences. He coined theterms endemic (for diseases usually foundin some places but not in others) andepidemic (for disease that are seen atsome times but not otherswww.drjayeshpatidar.blogspot.in
  4. 4. • One of the earliest theories on the origin of disease wasthat it was primarily the fault of human luxury. This wasexpressed by philosophers such as Plato andRousseau,and social critics like Jonathan Swift.• In the medieval Islamic world, physicians discovered thecontagious nature of infectious disease. In particular, thePersian physician Avicenna, considered a "father ofmodern medicine," in The Canon of Medicine (1020s),discovered the contagious nature of tuberculosis andsexually transmitted disease, and the distribution ofdisease through water and soil. Avicenna stated thatbodily secretion is contaminated by foul foreign earthlybodies before being infected.He introduced the methodof quarantine as a means of limiting the spread ofcontagious disease. He also used the method of riskfactor analysis, and proposed the idea of a syndrome inthe diagnosis of specific diseases.www.drjayeshpatidar.blogspot.in
  5. 5. • John Graunt, a professional haberdasher andserious amateur scientist, published Natural andPolitical Observations ... upon the Bills ofMortality in 1662. In it, he used analysis of themortality rolls in London before the Great Plagueto present one of the first life tables and reporttime trends for many diseases, new and old. Heprovided statistical evidence for many theorieson disease, and also refuted many widespreadideas on them.www.drjayeshpatidar.blogspot.in
  6. 6. • Dr. John Snow is famous for his investigations into thecauses of the 19th Century Cholera epidemics. Hebegan with noticing the significantly higher death rates intwo areas supplied by Southwark Company. Hisidentification of the Broad Street pump as the cause ofthe Soho epidemic is considered the classic example ofepidemiology. He used chlorine in an attempt to cleanthe water and had the handle removed, thus ending theoutbreak. (It has been questioned as to whether theepidemic was already in decline when Snow tookaction.) This has been perceived as a major event in thehistory of public health and can be regarded as thefounding event of the science of epidemiology.www.drjayeshpatidar.blogspot.in
  7. 7. Original map by Dr. John Snow showing the clusters of choleracases in the London epidemic of 1854www.drjayeshpatidar.blogspot.in
  8. 8. When the Black Death (bubonicplague) reached Al Andalus in the14th century, Ibn Khatimahypothesized that infectiousdiseases are caused by small"minute bodies" which enter thehuman body and cause disease.www.drjayeshpatidar.blogspot.in
  9. 9. • Another 14th century Andalusian-Arabian physician, Ibnal-Khatib (1313–1374), wrote a treatise called On thePlague, in which he stated how infectious disease canbe transmitted through bodily contact and "throughgarments, vessels and earrings.• In the middle of the 16th century, a famous Italian doctorfrom Verona named Girolamo Fracastoro was the first topropose a theory that these very small, unseeable,particles that cause disease were alive. They wereconsidered to be able to spread by air, multiply bythemselves and to be destroyable by fire.www.drjayeshpatidar.blogspot.in
  10. 10. • In this way he refuted Galens theory of miasms(poison gas in sick people).• In 1543 he wrote a book De contagione etcontagiosis morbis, in which he was the first topromote personal and environmental hygiene toprevent disease.• The development of a sufficiently powerfulmicroscope by Anton van Leeuwenhoek in 1675provided visual evidence of living particlesconsistent with a germ theory of disease.www.drjayeshpatidar.blogspot.in
  11. 11. • In the study of communicable and non-communicable diseases, the work ofepidemiologists ranges from outbreakinvestigation to study design, data collection andanalysis including the development of statisticalmodels to test hypotheses and thedocumentation of results for submission to peer-reviewed journals. Epidemiologists rely on anumber of other scientific disciplines, such asbiology (to better understand diseaseprocesses), Geographic Information Science (tostore data and map disease patterns) and socialscience disciplines (to better understandproximate and distal risk factors).www.drjayeshpatidar.blogspot.in
  12. 12. • Other pioneers include Danish physician P. A.Schleisner, who in 1849 related his work on theprevention of the epidemic of tetanus neonatorum on theVestmanna Islands in Iceland• Another important pioneer was Hungarian physicianIgnaz Semmelweis, who in 1847 brought down infantmortality at a Vienna hospital by instituting a disinfectionprocedure. His findings were published in 1850, but hiswork was ill received by his colleagues, whodiscontinued the procedure. Disinfection did not becomewidely practiced until British surgeon Joseph Listerdiscovered antiseptics in 1865 in light of the work ofLouis Pasteur.www.drjayeshpatidar.blogspot.in
  13. 13. • In the early 20th century, mathematical methodswere introduced into epidemiology by RonaldRoss, Anderson Gray McKendrick and others.• Another breakthrough was the 1954 publicationof the results of a British Doctors Study, led byRichard Doll and Austin Bradford Hill, which lentvery strong statistical support to the suspicionthat tobacco smoking was linked to lung cancer.www.drjayeshpatidar.blogspot.in
  14. 14. • Although epidemiology is sometimes viewed as acollection of statistical tools used to elucidate theassociations of exposures to health outcomes, a deeperunderstanding of this science is that of discoveringcausal relationships.• It is nearly impossible to say with perfect accuracy howeven the most simple physical systems behave beyondthe immediate future, much less the complex field ofepidemiology, which draws on biology, sociology,mathematics, statistics, anthropology, psychology, andpolicy; "Correlation does not imply causation" is acommon theme for much of the epidemiological literaturewww.drjayeshpatidar.blogspot.in
  15. 15. • For epidemiologists, the key is in the terminference. Epidemiologists use gathereddata and a broad range of biomedical andpsychosocial theories in an iterative way togenerate or expand theory, to testhypotheses, and to make educated,informed assertions about whichrelationships are causal, and about exactlyhow they are causalwww.drjayeshpatidar.blogspot.in
  16. 16. • Epidemiologists Rothman and Greenlandemphasize that the "one cause - oneeffect" understanding is a simplistic mis-belief. Most outcomes, whether disease ordeath, are caused by a chain or webconsisting of many component causeswww.drjayeshpatidar.blogspot.in
  17. 17. Epidemiologyis the study of factors affecting thehealth and illness of populations,and serves as the foundation andlogic of interventions made in theinterest of public health andpreventive medicine
  18. 18. • It is considered a cornerstonemethodology of public health research,and is highly regarded in evidence-basedmedicine for identifying risk factors fordisease and determining optimal treatmentapproaches to clinical practice.www.drjayeshpatidar.blogspot.in
  19. 19. • Epidemiological studies can only go to prove that an agent couldhave caused, but not that it did cause, an effect in any particularcase:• "Epidemiology is concerned with the incidenceof disease in populations and does not addressthe question of the cause of an individual’sdisease. This question, sometimes referred to asspecific causation, is beyond the domain of thescience of epidemiology. Epidemiology has itslimits at the point where an inference is madethat the relationship between an agent and adisease is causal (general causation) and wherethe magnitude of excess risk attributed to theagent has been determined; that is,epidemiology addresses whether an agent cancause a disease, not whether an agent didcause a specific plaintiff’s disease."www.drjayeshpatidar.blogspot.in
  20. 20. • As a public health discipline, epidemiologicevidence is often used to advocate bothpersonal measures like diet change andcorporate measures like removal of junkfood advertising, with study findingsdisseminated to the general public in orderto help people to make informed decisionsabout their healthwww.drjayeshpatidar.blogspot.in
  21. 21. • Epidemiological tools have provedeffective in establishing major causes ofdiseases like cholera and lung cancer buthave had problems with more subtlehealth issues, and several recentepidemiological results on medicaltreatments (for example, on the effects ofhormone replacement therapy) have beenrefuted by later randomized controlledtrialswww.drjayeshpatidar.blogspot.in
  22. 22. Population-based healthmanagement• Epidemiological practice and the results ofepidemiological analysis make asignificant contribution to emergingpopulation-based health managementframeworkswww.drjayeshpatidar.blogspot.in
  23. 23. • Population-based health managementencompasses the ability to:• Assess the health states and health needs of atarget population;• Implement and evaluate interventions that aredesigned to improve the health of thatpopulation; and• Efficiently and effectively provide care formembers of that population in a way that isconsistent with the community’s cultural, policyand health resource values.www.drjayeshpatidar.blogspot.in
  24. 24. • Modern population-based healthmanagement is complex, requiring amultiple set of skills (medical, political,technological, mathematical etc.) of whichepidemiological practice and analysis is acore component, that is unified withmanagement science to provide efficientand effective health care and healthguidance to a population.www.drjayeshpatidar.blogspot.in
  25. 25. • This task requires the forward looking ability ofmodern risk management approaches thattransform health risk factors, incidence,prevalence and mortality statistics (derived fromepidemiological analysis) into managementmetrics that not only guide how a health systemresponds to current population health issues, butalso how a health system can be managed tobetter respond to future potential populationhealth issues.www.drjayeshpatidar.blogspot.in
  26. 26. Legal interpretation• Epidemiological studies can only go to prove that an agent couldhave caused, but not that it did cause, an effect in any particularcase:• "Epidemiology is concerned with the incidence of disease inpopulations and does not address the question of the cause of anindividual’s disease. This question, sometimes referred to asspecific causation, is beyond the domain of the science ofepidemiology. Epidemiology has its limits at the point where aninference is made that the relationship between an agent and adisease is causal (general causation) and where the magnitude ofexcess risk attributed to the agent has been determined; that is,epidemiology addresses whether an agent can cause a disease, notwhether an agent did cause a specific plaintiff’s disease."[13]www.drjayeshpatidar.blogspot.in
  27. 27. • In United States law, epidemiology alonecannot prove that a causal associationdoes not exist in general. Conversely, itcan be (and is in some circumstances)taken by US courts, in an individual case,to justify an inference that a causalassociation does exist, based upon abalance of probability.www.drjayeshpatidar.blogspot.in
  28. 28. Population-based healthmanagement• Epidemiological practice and the results ofepidemiological analysis make a significant contributionto emerging population-based health managementframeworks.• Population-based health management encompasses theability to:• Assess the health states and health needs of a targetpopulation;• Implement and evaluate interventions that are designedto improve the health of that population; and• Efficiently and effectively provide care for members ofthat population in a way that is consistent with thecommunity’s cultural, policy and health resource values.www.drjayeshpatidar.blogspot.in
  29. 29. Modern population-based health management is complex,requiring a multipleset of skills (medical, political, technological,mathematical etc.) of which epidemiological practice andanalysis is a corecomponent, that is unified with managementscience to provide efficient and effective health care and healthguidance to a population.This task requires the forward looking ability of modern riskmanagement approaches that transform health risk factors,incidence, prevalence and mortality statistics(derived from epidemiological analysis) into managementmetrics that not only guide howa health system responds to current population health issues,but also how a healthsystem can be managed to better respondto future potential population health issues.www.drjayeshpatidar.blogspot.in
  30. 30. Advocacy• As a public health discipline, epidemiologic evidence is often used toadvocate both personal measures like diet change and corporatemeasures like removal of junk food advertising, with study findingsdisseminated to the general public in order to help people to makeinformed decisions about their health. Often the uncertainties aboutthese findings are not communicated well; news articles oftenprominently report the latest result of one study with little mention ofits limitations, caveats, or context. Epidemiological tools haveproved effective in establishing major causes of diseases likecholera and lung cancer but have had problems with more subtlehealth issues, and several recent epidemiological results on medicaltreatments (for example, on the effects of hormone replacementtherapy) have been refuted by later randomized controlled trials.www.drjayeshpatidar.blogspot.in
  31. 31. Sources of data• Routinely collected data• Records from health sector• original documentswww.drjayeshpatidar.blogspot.in
  32. 32. Descriptive epidemiologyMigrant studiesPerson distribution•Defining persons who develop the disease byage,sex,occupation,marital status ,social class and other hostfactors•Dont necessarily represent etiological factors ,but theycontribute a good deal to our understanding of the naturalhistory of disease .Age: strongly related to disease than any other single hostfactor .Certain diseases –more frequent in certain age groups than inothers –
  33. 33. eg.measles in childhoodcancer in middle age atheroslerosisin old ageAs age advances –many chronic progressive disorders incresein prevalence.BimodalityTher may be two separate peaks instead of one in the ageincidence curve of a disease .eg leukemia,Hodgkins disease.
  34. 34. gender• Is a host characteristic which is often studied in relation to disease• Indexes used-sex ratio,sex-specific morbidity&mortality rates• Some diseases common in women:diabetes ,hyperthyroidism and obesityUncommon ’lung cancer,chd1. Basic biological differences between the sexes including sex linked geneticinheritance2. Cultural and behavioral differences between the sexes (alcoholism,automobile useetc.)due to different roleswww.drjayeshpatidar.blogspot.in
  35. 35. •ethnicityDifferences in disease occurrence observed between populationsubgroupsof different racial and ethnic origin.• MARITAL STAUSMortality rates were lower for males and females who are married.•Occupation•Social class•Upperer class•behaviorCigarette smoking alcoholismSedentary life style•Stress•Migrationwww.drjayeshpatidar.blogspot.in
  36. 36. Measurement of diseaseDisease load –mortality,morbidity disability etcMortality –straight forwardMorbidity –2 aspects—incidence and prevalenceMeasurement can be extended to health statesandevents.www.drjayeshpatidar.blogspot.in
  37. 37. Comparing with known indicescomparison between different populations and subgroups of he same population,it is possible to arrive at clues to disease etiology .Formulation of a hypothesisRelating to disease etiologyShould specify the following1. The population2. The specific cause being considered3. The expected outcome4. The dose response relationship5. The time response relationshipEg. The smoking of 30-40cigaretes per day causes lung cancer in 10% of smokersafter 20 years of exposure.www.drjayeshpatidar.blogspot.in
  38. 38. Uses of descriptive epidemiology1. Provide data regarding magnitude of the disease load and the type ofdiseases problems in he community in terms of morbidity and mortalityrates and ratios2. Provide clues to disease etiology and help in the formulation of anetiological hypothesis3. Provide back ground data for planning ,organising and evaluatingpreventive and curative services.4. they contribute to research by describing variations in disease occurrenceby time ,place and person.www.drjayeshpatidar.blogspot.in
  39. 39. Analytical epidemiology• The subject of interest is individual withinthe population• The object is to test hypothesis• 2 distinct types of obsnl. Studies1. Case control studies2. Cohort studieswww.drjayeshpatidar.blogspot.in
  40. 40. • From each of these one can determine• A. whether or not a statistical associationexists between a disease and a suspectedfactor• If one exists –the strength of associationwww.drjayeshpatidar.blogspot.in
  41. 41. Analytical epidemiology (Schematic diagram)Case control study indls with particular disease}casesFactors indls without particular disease}controlsPresentOrAbsentPROSPECTIVE(cohort study)Indl. Exposed to particular factors.Inld unexposed to particular factorspresence or absence of particular diseaseTimewww.drjayeshpatidar.blogspot.in
  42. 42. Case control study(retrospective study)3 distinct features• Both exposure and outcome haveoccurred before the start of the study• It uses a control/comparison group• The study proceeds backwards fromeffect to cause.www.drjayeshpatidar.blogspot.in
  43. 43. • Involves 2 populations• Cases controlswww.drjayeshpatidar.blogspot.in
  44. 44. basic steps• Selection of cases and control• Matching• Measurement of exposure• Analysis and interpretationwww.drjayeshpatidar.blogspot.in
  45. 45. Selection of cases and control1. Selection of cases:a. Definition of case-involves 2specifications—i. Diagnostic criteria: criteria of the diseaseand stage of the disease if anyii. Eligibility criteria :only newly diagnosedcases within a specified period of timeare eligiblewww.drjayeshpatidar.blogspot.in
  46. 46. b.Sources of cases• Hospitals :from a single or network ofhospitals admitted during a specifiedperiod of time.• general population: all cases of studydiseases occurring within a definedgeographic area during a specified periodof time are ascertained—often through asurvey or ,a disease registry or hospitalnetwork.www.drjayeshpatidar.blogspot.in
  47. 47. Selection of controls• Controls must be free from the diseaseunder study.• They must be similar to the cases aspossible ,except for the absence of thedisease under studywww.drjayeshpatidar.blogspot.in
  48. 48. Sources of controls• Hospitals• Relatives• Neighbors• general populationwww.drjayeshpatidar.blogspot.in
  49. 49. Matching• Comparability between cases and controlsto be ensured this is done through -Matching• Defined as the process by which we selectcontrols in such a way that they are similarto cases with regard to certain pertinentselected variables. which are known toinfluence the outcome of the disease andwhich if not adequately matched forcomparability ,could distort or confoundthe results.www.drjayeshpatidar.blogspot.in
  50. 50. Matching• A confounding factor is defined as the oneassociated with both exposure and disease, andis distributed unequally in the study and controlgroups.• Eg:Role of alcohol –esophageal cancerSmoking is a compounding factor---it is associatedwith the consumption of alcohol and it is anindependent risk factor for esophageal cancer.in this case the effect of alcohol consumptioncan be determined only if the influence ofsmoking is neutralized by matchingwww.drjayeshpatidar.blogspot.in
  51. 51. • Age could be compounding variable:Relationship between Steroid contraceptiveand breast cancerMatching protects against an un expectedstrong association between the matchingfactor and the disease .www.drjayeshpatidar.blogspot.in
  52. 52. Kinds of matching• Group• Pairswww.drjayeshpatidar.blogspot.in
  53. 53. Measurement of exposure• Information about exposure should beobtained in precisely the same mannerboth for cases and controls• May be 0btained by interviews,questionnaires or by studying pastrecords of cases such as hospital recordsor employment cardswww.drjayeshpatidar.blogspot.in
  54. 54. AnalysisTo find out• Exposure rates among cases and controlsto suspected factor• ESTIMATION OF DISEASE RISKASSOCIATED WITH EXPOSURE (ODDRATIO)www.drjayeshpatidar.blogspot.in
  55. 55. Exposure rates(FREQUENCY OFEXPOSURE)• Direct estimation of the exposure rates tosuspected factor in disease and nondisease group.www.drjayeshpatidar.blogspot.in
  56. 56. A case control study of smoking and lung CaCases(with lung Ca) Controls (without lungCa)Smokers <5 cigars/day 33(a) 55(b)Non smokers 2(c) 27(d)total 35(a+c) 82(b+d)Table 1Exposure ratesCases=a/a=c=33/35=94.2%Controls=b/b+d=55/82=67%,p<0.001Frequency rate of Lung Ca is higher among smokersthan non smokers.If p</=0.o5----statistically significantwww.drjayeshpatidar.blogspot.in
  57. 57. Estimation of risk• Estimation of disease risk associated withexposure—is obtained by an index knownas relative risk or risk ratio .Relative risk = incidence among exposedincidence among non exposed=a/(a+b) + c/(c+d)www.drjayeshpatidar.blogspot.in
  58. 58. Odds ratio(cross product ratio)(OR)• Measure of the strength of the assocn.between risk factor and outcome• Odds ratio is closely related to relative risk• The derivation of odds ratio is based on3asssumptions1. The diseases being investigated must berelatively rare2. The cases must be representative of thosewith the disease3. Controls must be representative withoutdiseasewww.drjayeshpatidar.blogspot.in
  59. 59. Suspected or riskfactorsCase /disease present Control /diseaseabsentpresent a bAbsentOddsratio=ad/bcca+cdb+dOdds ratio=ad/bc=33x27/55x2=8.1(data from table1)Smokers of less than 5 cigars/day showed a risk of having lung Ca 8.1times thatof nonsmokers. odd ratio is the key parameter in the analysis of case controlstudies www.drjayeshpatidar.blogspot.in
  60. 60. Bias in case control studies• Any systematic error in the determination of theassociation between the exposure and disease• Reflects non comparabilityBias— confounding rectified by matching memory or recall selection—cases may not be representative in generalpopulation Berkesonian –different rates of admission to hospitals forpeople with different diseases Interviewer-double blinding is used to rectifywww.drjayeshpatidar.blogspot.in
  61. 61. Advantages &disadvantagesAdvantages• Easy to carry out• Rapid and inexpensive• Require few subjects• Suitable to investigate rare diseases.• No risk to subjects• Study of different etiological factors• Risk factors can be identified• No attrition problems..• ethical problems minimalDisadvantage•Bias-on memory or pastrecords, accuracy may beuncertain•Selection of appropriatecontrol group difficult•Cannot measureincidence ,can onlymeasure relative risk•Don’t distinguish betweencause and associatedfactors•Concern of representativeness of cases and controlswww.drjayeshpatidar.blogspot.in
  62. 62. Cohortstudies(Longitudinal,prospective,incidence,forward- looking study)• Usually undertaken to obtain additional evidence torefute to or support of the existence of an associationbetween suspected causes and diseasesFeatures1. Cohorts are identified prior to the appearance of thedisease under investigation2. Study groups ,so defined ,are observed over a periodof time to determine the frequency of disease amongthem3. Study proceeds forward from cause to effectwww.drjayeshpatidar.blogspot.in
  63. 63. Concept• Cohort –a group of people who share a commoncharacteristic or experience within a defined time periodIndications for cohort study• When there is a good evidence of an associationbetween exposure and disease ,as derived from clinicalobservations and supported by descriptive and casecontrol studies• When exposure is rare ,but the incidence of the diseasehigh among exposed.• When attrition of study population can be minimized.• Ample funds are availablewww.drjayeshpatidar.blogspot.in
  64. 64. framework• Cause to effect =exposure has occurred but not thediseasecohort DiseaseYes nototalExposed toputativeetiologic factora b A+b(studycohort)not Exposed toputativeetiologic factorc d C+d(control cohort)www.drjayeshpatidar.blogspot.in
  65. 65. General considerations inassembling cohort• Free from disease• Both the groups equally susceptible• Both the groups should be comparable• Diagnostic eligibility criteria the disease must bedefined before handA well designed cohort study is considered themost reliable means of showing an associationbetween a suspected risk factor and subsequentdisease .www.drjayeshpatidar.blogspot.in
  66. 66. types• Prospective—the out come has not yetoccurred at the time of inv. begin in thepresent and continue in the future• Retrospective. outcomes haveoccuredbefore the inv.goes back in time• A combination of both: cohort from pastrecordsand assessed of dat efor outcomewww.drjayeshpatidar.blogspot.in
  67. 67. Elements• Selection of study subjects• Obtaining data on exposure• Selection of comparison of groups• Follow up• analysiswww.drjayeshpatidar.blogspot.in
  68. 68. Selection of study subjects• General population• Special groupsThese may be special groups or exposure groups that can readily bestudied1.Select groups: Professional groups(eg.doctors,nurses,lawyersetc.),insured persons, obstetric population, college alumini,govt employees,volunteers etc.These groups are homogenous, advantages of accessibility and easy tofollow up for a protracted period.2.Exposure groups: if the exposure is rare more economical procedure toselect a cohort of persons known to have experienced theexposure.eg.workers in industries.www.drjayeshpatidar.blogspot.in
  69. 69. Obtaining data on exposureInformation can be obtained from1. Cohort members:interviews,mail questionnaires2. Review of records: certain kind of information can be obtainedfrom records.eg.dose of radiation, kinds of surgery,detailas ofmedication3. Medical examination or special tests:eg. BP,ECG.4.Environmental surveys: obtaining information on exposure levels ofthe suspected factor in the environment where the cohort lived orworked.Information to be collected in a manner that will allow classification ofcohorta. According to whether or not they have been exposed to thesuspected factor or not.b.According to the degree of exposureIn addition demographic variables also to be collected.www.drjayeshpatidar.blogspot.in
  70. 70. Selection of comparison groups1. Internal comparisons:no outside comparison groups.in built comparison groups.ie.single cohort enters the study ,and its members may ,onthe basis of information obtained, be classified intoseveral comparison groups according to the level ofexposure to risk before the development of the diseasein question.eg.smoking ,blood pressure, serumcholesterolThe groups so defined, are compared in terms of theirsubsequent morbidity and mortality rates.www.drjayeshpatidar.blogspot.in
  71. 71. Age standardized death rates /100000men per year byamount of current smokingClassification ofexposure (cigarettes)No of deaths Death rate½ pack 24 95.21/2pack-1 pack 84 107.81-2 pack 90 229.22packs+ 97 264.2Mortality from lung Ca increases with increased no of cigarettes smoked –thereis valid associationwww.drjayeshpatidar.blogspot.in
  72. 72. b.External comparisons• External control Is used when informationon degree on exposure is not available –evaluate the experience of the exposedgroup.Eg .smokers and non smokers ,a cohort ofradiologist with a cohort ofophthalmologist.study and control variables should besimilar in demographic and possiblyimportant variableswww.drjayeshpatidar.blogspot.in
  73. 73. c.Comparison group with general population values• None available –mortality experience ofthe exposed group is compared with themortality experience of the generalpopulation• Eg. Comparison of frequency of lungca.among uranium workers with lungca.mortality with general population wherethe miners resided.www.drjayeshpatidar.blogspot.in
  74. 74. d.Follow up• Regular follow up is part of cohortstudies.Procedure to obtain data:1. Periodic medical examination of each memberof the cohort2. Reviewing physician and hospital records3. Routine surveillance of death records4. Mailed questionnaire ,telephone call s,periodichome visits-preferably all 3 on annual basiswww.drjayeshpatidar.blogspot.in
  75. 75. analysisThe data are analysed in terms of1. Incidence rates of outcome amongexposed and non exposed2. Estimation of riskwww.drjayeshpatidar.blogspot.in
  76. 76. Incidence rates• Can be calculated directlyCigarette smoking Developed lung ca Did not developlung catotalyes 70(a) 6930(b) 7000(a+b)no 3(c) 2997(d) 3000(c+d)Incidence ratesa. among smokers =70/7000=10/1000b. Among non smokers=3/3000=1/1000p<0.001Contingency table applied to hypothetical cigarette smoking andlung cancer eg.www.drjayeshpatidar.blogspot.in
  77. 77. Estimation of risksRisk of outcome :• relative risk• attributable ratiorelative risk(RR)= incidence of disease /deathamong exposedincidence disease/death among nonexposedRR=10/1=10(data from table)relative risk (RR) is a direct measure or (index) of the strength of the associationbetween the cause and effectRR-1-indicates no association>1suggests +association between exposure and the disease under study2-indicates-incidence rate of disease is 2 times higher in ht exposed group ascompared to un exposedSmokers are 10 times at greater risk of developing lung ca than non smokersLarger RR---greater the strength of associationwww.drjayeshpatidar.blogspot.in
  78. 78. attributable risk (AR)(risk difference)• Difference in incidence rate of disease or death between anexposed and non exposed group• Expressed in %• AR=incidence of disease rate among exposed-incidence disease rateamong nonexposedincidence of disease rate among exposedAR=10-1/10x100=90%(table data)Indicates to what extend the disease under study can be attributed tothe exposure.www.drjayeshpatidar.blogspot.in
  79. 79. Population of attributable risk• Incidence of the disease or death in the total population - Incidence ofthe disease or death among those who were not exposed to thesuspected causal factorDeaths per 100, 000-yearsHeavy smokers 224 Exposed to suspectedfactor(a)Non smokers 10 Non exposed tosuspected factors(b)Deaths in totalpopulation74(c)Individual RR=a/b=224/10=22.4Population AR=c-b/cx100=86%Deaths per 100, 000-yearsHeavy smokers 224 Exposed to suspectedfactor(a)Non smokers 10 Non exposed tosuspected factors(b)Deaths in totalpopulation74(c)Individual RR=a/b=224/10=22.4Population AR=c-b/cx100=86%www.drjayeshpatidar.blogspot.in
  80. 80. • The concept of population attributable riskis useful in that it provides an estimate ofthe amount by which the disease could bereduced in that population if the suspectedfactor was eliminated or modified• In the eg.--86%deaths could have beenavoided if the risk of cigarettes wereeliminated.www.drjayeshpatidar.blogspot.in
  81. 81. Relative risk and attributable risk• RR is imp. in etiological enquiries.• its size is a better index for assessing theetiological role of a factor in disease.• The larger the RR ,the stronger theassocn.between cause and effect.• RR does not reflect the potential public healthimportance as does the attributable risk• AR gives a better idea than does RR of theimpact of successful preventive or public healthprogramme might have in reducing the problemwww.drjayeshpatidar.blogspot.in
  82. 82. Cardiovascular risk100,000patient yearsAges30-39 40-44RR 2.8 2.8AR 3.5 20.0The RR and AR of cardiovascular complications in women taking oralcontraceptivesRR independent of ageAR is .5times higher in the older age.This epidemiological observation is the basis for not recommendingOCP to those aged 35yers and abovewww.drjayeshpatidar.blogspot.in
  83. 83. Risk assessment smokers vs non smokersCause ofdeathDeath rate/1000Smokers nonsmokersRR ARLung Ca .90 0.7 12.86 92.2CHD 4.87 4.22 1.15 13. 3Smoking is attributable to 92%of Calung&13.3%of CHD .In CHD,both RR and AR not very high –suggests not much of thedisease could be prevented as compared to lung cawww.drjayeshpatidar.blogspot.in
  84. 84. Advantages &disadvantages1. Incidence can be calculated2. Several possible outcomesrelated to exposure can bestudied simultaneously3. Provide direct estimate of RR4. Dose response ratio –can becalculated5. Bias can be minimized-groupsformed before disease develops1. Long time2. Large no of people3. Administrative problems-loss ofexperienced staff ,loss offunding and extensive recordkeeping are in evitable4. Original cohort may be lo5. Selection of controls limitingfactor6. There may be change indiagnostic criteria7. Expensive8. May alter peoples behavior9. Limited no of factors areconcentratedwww.drjayeshpatidar.blogspot.in
  85. 85. Main difference between case control &cohort1. Proceeds from effect to cause2. Starts with disease3. Tests whether the diseaseoccurs more frequently in thosewith disease than among thosewithout disease4. First Approach to testinghypothesis5. Involve fewer no of subjects6. quick results7. Suitable for the study of raredisease8. Only estimate of RR9. Cannot yield information aboutdisease other than that selectedfor study10. Relatively in expensive1. cause to effect2. People3. Test in those exposed4. Reserved for testing preciselyformulated hypothesis5. Larger no of subjects6. Long follow up period7. Inappropriate for rare disease8. Yields incidence rate,RR&AR9. Can yield information about morethan one disease outcome10. expensivewww.drjayeshpatidar.blogspot.in
  86. 86. Experimental epidemiology• Equated with randomised controls• It is similar to cohort study except The conditions underwhich study is carried out is under the direct control ofthe investigator• Involve action, intervention or manipulation –deliberateapplication or withdrawal of the suspected causes orchanging one variable in the causative chain in theexperimental group while making no change in thecontrol group.• Observing and comparing the outcome of the experimentin both ht egroupswww.drjayeshpatidar.blogspot.in
  87. 87. Aims1. To provide scientific proof of etiologic factor which may permit themodification or control of those diseases2. To provide a method of measuring the effectiveness andefficiency of health services for he prevention ,control andtreatment of disease and improve the health of the communityThey may be conducted in animals and human beingswww.drjayeshpatidar.blogspot.in
  88. 88. Animal studies-application1. Experimental reproduction of human diseasein animals to confirm etiological hypothesesand to study their pathogenetic phenomena ormechanisms2. Testing the efficacy-preventive andtherapeutic measures such as vaccines anddrugs3. Completing a natural history of disease.www.drjayeshpatidar.blogspot.in
  89. 89. Animal experimentsAdvantages• Can be bred in labs andmanipulated easily• They multiply rapidly andgenetic studies can becarried out• Disadvantages• Not all human diseasescan be reproduced inanimals• conclusions may not bestrictly applicable tohuman beingswww.drjayeshpatidar.blogspot.in
  90. 90. human experiments• Will always have to investigate disease etiology and toevaluate the preventive and therapeutic measures.• Even more in essential in the investigation of diseasesthat cannot be reproduced in animals• The benefits of the experiment have to be weighedagainst risk involved. WHO has laid down strict code ofpractice in connection with human trialswww.drjayeshpatidar.blogspot.in
  91. 91. types• Randomised controlled trials-thoseinvolving a process of random allocation• Non randomised or non experimentaltrialswww.drjayeshpatidar.blogspot.in
  92. 92. Randomised controlled trialsdesignSelect suitable population(ref.or target)Select suitable sampleMake necessary exclusionsrandomiseExperiment group ControlManipulation and follow upThose not eligibleDo not wish to give consentAssessmentwww.drjayeshpatidar.blogspot.in
  93. 93. Steps in conducting RCT• Developing a protocol• Selecting reference and experimentalpopulations• Randomisation• Manipulation and intervention• Follow up• Assessment of outcomewww.drjayeshpatidar.blogspot.in
  94. 94. The protocol• Aims and objectives• Questions to be answered• Criteria for the selection of study and control groups• Size of the sample• The procedures for allocation of subj.into study andcontrol groups treatment to be appliedProtocol to be strictly adhered till end ,helps in preventing bias andsource of error in the studyPilot/preliminary studies are done prior to protocol –feasibility/operational efficiency of certain procedures, or unknowneffects ,or on the acceptability of certain policieswww.drjayeshpatidar.blogspot.in
  95. 95. Selecting reference and experimental populations1. Reference or target population The popln.to which the findings of the trial ,if found successful areexpected to be applicable Mankind/geographically limited/person in specificage/gender/occupational/social group. Population of a whole city, school children,etc.2.experimental /study populations Derived from ref popln.-actual popln. Ideally should be randomly chosen. 3 criteria to be fulfilled:informed consentRepresentative of the population to which they belongQualified or eligible for the trialwww.drjayeshpatidar.blogspot.in
  96. 96. 3.Randomisation Statistical procedureControl study• Helps in removing bias and allow for comparability• Heart of control trialwww.drjayeshpatidar.blogspot.in
  97. 97. Manipulation• Deliberate application or withdrawal orreduction of the suspected causal factoras laid down in protocol• Creates independentvariable(drug,vaccinedietary component ,a habit etc.) whoseeffect is determined by measurement ofthe final outcome –dependent variable(incidence of disease, survival time,recovery period)www.drjayeshpatidar.blogspot.in
  98. 98. Follow up• Examination of experimental and controlgroup at defined intervals time, in astandard manner ,with equal intensity,under the same given circumstances inthe same time frame till final assessmentof outcome• There can be attritionwww.drjayeshpatidar.blogspot.in
  99. 99. Assessment• Positive results :benefits of experimentalmeasure• Negative results severity and frequencyof side effects and complicationsBias1. Part of participants –subject variation2. Observer bias-3. Evaluation to rectify blinding is used.www.drjayeshpatidar.blogspot.in
  100. 100. Study designs -controlled• Concurrent parallel :comparisons are made between 2 randomly assignedgroups one exposed to treatment other not• Cross over type :each pt.serves as his own controlRandomisedClinical trialsPreventive trialsRisk factorCessation experimentsTrial of etiological agentsEvaluation of health serviceswww.drjayeshpatidar.blogspot.in
  101. 101. Nonrandomized trials• Uncontrolled trials• Natural experiments• Before and after comparison studieswww.drjayeshpatidar.blogspot.in