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Epidemiology
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Epidemiology

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  • 1. EPIDEMIOLOGY www.drjayeshpatidar.blogspot.com www.drjayeshpatidar.blogspot.in
  • 2. • Epidemiology, "the study of what is upon the people", is derived from the Greek terms epi = upon, among; demos = people, district; logos = study, word, discourse; suggesting that it applies only to human populations. But www.drjayeshpatidar.blogspot.in
  • 3. History • The Greek physician Hippocrates is sometimes said to be the uncle of epidemiology. He is the first person known to have examined the relationships between the occurrence of disease and environmental influences. He coined the terms endemic (for diseases usually found in some places but not in others) and epidemic (for disease that are seen at some times but not others www.drjayeshpatidar.blogspot.in
  • 4. • One of the earliest theories on the origin of disease was that it was primarily the fault of human luxury. This was expressed by philosophers such as Plato and Rousseau,and social critics like Jonathan Swift. • In the medieval Islamic world, physicians discovered the contagious nature of infectious disease. In particular, the Persian physician Avicenna, considered a "father of modern medicine," in The Canon of Medicine (1020s), discovered the contagious nature of tuberculosis and sexually transmitted disease, and the distribution of disease through water and soil. Avicenna stated that bodily secretion is contaminated by foul foreign earthly bodies before being infected.He introduced the method of quarantine as a means of limiting the spread of contagious disease. He also used the method of risk factor analysis, and proposed the idea of a syndrome in the diagnosis of specific diseases. www.drjayeshpatidar.blogspot.in
  • 5. • John Graunt, a professional haberdasher and serious amateur scientist, published Natural and Political Observations ... upon the Bills of Mortality in 1662. In it, he used analysis of the mortality rolls in London before the Great Plague to present one of the first life tables and report time trends for many diseases, new and old. He provided statistical evidence for many theories on disease, and also refuted many widespread ideas on them. www.drjayeshpatidar.blogspot.in
  • 6. • Dr. John Snow is famous for his investigations into the causes of the 19th Century Cholera epidemics. He began with noticing the significantly higher death rates in two areas supplied by Southwark Company. His identification of the Broad Street pump as the cause of the Soho epidemic is considered the classic example of epidemiology. He used chlorine in an attempt to clean the water and had the handle removed, thus ending the outbreak. (It has been questioned as to whether the epidemic was already in decline when Snow took action.) This has been perceived as a major event in the history of public health and can be regarded as the founding event of the science of epidemiology. www.drjayeshpatidar.blogspot.in
  • 7. Original map by Dr. John Snow showing the clusters of cholera cases in the London epidemic of 1854www.drjayeshpatidar.blogspot.in
  • 8. When the Black Death (bubonic plague) reached Al Andalus in the 14th century, Ibn Khatima hypothesized that infectious diseases are caused by small "minute bodies" which enter the human body and cause disease. www.drjayeshpatidar.blogspot.in
  • 9. • Another 14th century Andalusian-Arabian physician, Ibn al-Khatib (1313–1374), wrote a treatise called On the Plague, in which he stated how infectious disease can be transmitted through bodily contact and "through garments, vessels and earrings. • In the middle of the 16th century, a famous Italian doctor from Verona named Girolamo Fracastoro was the first to propose a theory that these very small, unseeable, particles that cause disease were alive. They were considered to be able to spread by air, multiply by themselves and to be destroyable by fire. www.drjayeshpatidar.blogspot.in
  • 10. • In this way he refuted Galen's theory of miasms (poison gas in sick people). • In 1543 he wrote a book De contagione et contagiosis morbis, in which he was the first to promote personal and environmental hygiene to prevent disease. • The development of a sufficiently powerful microscope by Anton van Leeuwenhoek in 1675 provided visual evidence of living particles consistent with a germ theory of disease. www.drjayeshpatidar.blogspot.in
  • 11. • In the study of communicable and non- communicable diseases, the work of epidemiologists ranges from outbreak investigation to study design, data collection and analysis including the development of statistical models to test hypotheses and the documentation of results for submission to peer- reviewed journals. Epidemiologists rely on a number of other scientific disciplines, such as biology (to better understand disease processes), Geographic Information Science (to store data and map disease patterns) and social science disciplines (to better understand proximate and distal risk factors). www.drjayeshpatidar.blogspot.in
  • 12. • Other pioneers include Danish physician P. A. Schleisner, who in 1849 related his work on the prevention of the epidemic of tetanus neonatorum on the Vestmanna Islands in Iceland • Another important pioneer was Hungarian physician Ignaz Semmelweis, who in 1847 brought down infant mortality at a Vienna hospital by instituting a disinfection procedure. His findings were published in 1850, but his work was ill received by his colleagues, who discontinued the procedure. Disinfection did not become widely practiced until British surgeon Joseph Lister 'discovered' antiseptics in 1865 in light of the work of Louis Pasteur. www.drjayeshpatidar.blogspot.in
  • 13. • In the early 20th century, mathematical methods were introduced into epidemiology by Ronald Ross, Anderson Gray McKendrick and others. • Another breakthrough was the 1954 publication of the results of a British Doctors Study, led by Richard Doll and Austin Bradford Hill, which lent very strong statistical support to the suspicion that tobacco smoking was linked to lung cancer. www.drjayeshpatidar.blogspot.in
  • 14. • Although epidemiology is sometimes viewed as a collection of statistical tools used to elucidate the associations of exposures to health outcomes, a deeper understanding of this science is that of discovering causal relationships. • It is nearly impossible to say with perfect accuracy how even the most simple physical systems behave beyond the immediate future, much less the complex field of epidemiology, which draws on biology, sociology, mathematics, statistics, anthropology, psychology, and policy; "Correlation does not imply causation" is a common theme for much of the epidemiological literature www.drjayeshpatidar.blogspot.in
  • 15. • For epidemiologists, the key is in the term inference. Epidemiologists use gathered data and a broad range of biomedical and psychosocial theories in an iterative way to generate or expand theory, to test hypotheses, and to make educated, informed assertions about which relationships are causal, and about exactly how they are causal www.drjayeshpatidar.blogspot.in
  • 16. • Epidemiologists Rothman and Greenland emphasize that the "one cause - one effect" understanding is a simplistic mis- belief. Most outcomes, whether disease or death, are caused by a chain or web consisting of many component causes www.drjayeshpatidar.blogspot.in
  • 17. Epidemiology is the study of factors affecting the health and illness of populations, and serves as the foundation and logic of interventions made in the interest of public health and preventive medicine
  • 18. • It is considered a cornerstone methodology of public health research, and is highly regarded in evidence-based medicine for identifying risk factors for disease and determining optimal treatment approaches to clinical practice. www.drjayeshpatidar.blogspot.in
  • 19. • Epidemiological studies can only go to prove that an agent could have caused, but not that it did cause, an effect in any particular case: • "Epidemiology is concerned with the incidence of disease in populations and does not address the question of the cause of an individual’s disease. This question, sometimes referred to as specific causation, is beyond the domain of the science of epidemiology. Epidemiology has its limits at the point where an inference is made that the relationship between an agent and a disease is causal (general causation) and where the magnitude of excess risk attributed to the agent has been determined; that is, epidemiology addresses whether an agent can cause a disease, not whether an agent did cause a specific plaintiff’s disease." www.drjayeshpatidar.blogspot.in
  • 20. • As a public health discipline, epidemiologic evidence is often used to advocate both personal measures like diet change and corporate measures like removal of junk food advertising, with study findings disseminated to the general public in order to help people to make informed decisions about their health www.drjayeshpatidar.blogspot.in
  • 21. • Epidemiological tools have proved effective in establishing major causes of diseases like cholera and lung cancer but have had problems with more subtle health issues, and several recent epidemiological results on medical treatments (for example, on the effects of hormone replacement therapy) have been refuted by later randomized controlled trials www.drjayeshpatidar.blogspot.in
  • 22. Population-based health management • Epidemiological practice and the results of epidemiological analysis make a significant contribution to emerging population-based health management frameworks www.drjayeshpatidar.blogspot.in
  • 23. • Population-based health management encompasses the ability to: • Assess the health states and health needs of a target population; • Implement and evaluate interventions that are designed to improve the health of that population; and • Efficiently and effectively provide care for members of that population in a way that is consistent with the community’s cultural, policy and health resource values. www.drjayeshpatidar.blogspot.in
  • 24. • Modern population-based health management is complex, requiring a multiple set of skills (medical, political, technological, mathematical etc.) of which epidemiological practice and analysis is a core component, that is unified with management science to provide efficient and effective health care and health guidance to a population. www.drjayeshpatidar.blogspot.in
  • 25. • This task requires the forward looking ability of modern risk management approaches that transform health risk factors, incidence, prevalence and mortality statistics (derived from epidemiological analysis) into management metrics that not only guide how a health system responds to current population health issues, but also how a health system can be managed to better respond to future potential population health issues. www.drjayeshpatidar.blogspot.in
  • 26. Legal interpretation • Epidemiological studies can only go to prove that an agent could have caused, but not that it did cause, an effect in any particular case: • "Epidemiology is concerned with the incidence of disease in populations and does not address the question of the cause of an individual’s disease. This question, sometimes referred to as specific causation, is beyond the domain of the science of epidemiology. Epidemiology has its limits at the point where an inference is made that the relationship between an agent and a disease is causal (general causation) and where the magnitude of excess risk attributed to the agent has been determined; that is, epidemiology addresses whether an agent can cause a disease, not whether an agent did cause a specific plaintiff’s disease."[13] www.drjayeshpatidar.blogspot.in
  • 27. • In United States law, epidemiology alone cannot prove that a causal association does not exist in general. Conversely, it can be (and is in some circumstances) taken by US courts, in an individual case, to justify an inference that a causal association does exist, based upon a balance of probability. www.drjayeshpatidar.blogspot.in
  • 28. Population-based health management • Epidemiological practice and the results of epidemiological analysis make a significant contribution to emerging population-based health management frameworks. • Population-based health management encompasses the ability to: • Assess the health states and health needs of a target population; • Implement and evaluate interventions that are designed to improve the health of that population; and • Efficiently and effectively provide care for members of that population in a way that is consistent with the community’s cultural, policy and health resource values. www.drjayeshpatidar.blogspot.in
  • 29. Modern population-based health management is complex, requiring a multipleset of skills (medical, political, technological, mathematical etc.) of which epidemiological practice and analysis is a corecomponent, that is unified with management science to provide efficient and effective health care and health guidance to a population. This task requires the forward looking ability of modern risk management approaches that transform health risk factors, incidence, prevalence and mortality statistics (derived from epidemiological analysis) into management metrics that not only guide how a health system responds to current population health issues, but also how a healthsystem can be managed to better respond to future potential population health issues. www.drjayeshpatidar.blogspot.in
  • 30. Advocacy • As a public health discipline, epidemiologic evidence is often used to advocate both personal measures like diet change and corporate measures like removal of junk food advertising, with study findings disseminated to the general public in order to help people to make informed decisions about their health. Often the uncertainties about these findings are not communicated well; news articles often prominently report the latest result of one study with little mention of its limitations, caveats, or context. Epidemiological tools have proved effective in establishing major causes of diseases like cholera and lung cancer but have had problems with more subtle health issues, and several recent epidemiological results on medical treatments (for example, on the effects of hormone replacement therapy) have been refuted by later randomized controlled trials. www.drjayeshpatidar.blogspot.in
  • 31. Sources of data • Routinely collected data • Records from health sector • original documents www.drjayeshpatidar.blogspot.in
  • 32. Descriptive epidemiology Migrant studies Person distribution •Defining persons who develop the disease by age,sex,occupation,marital status ,social class and other host factors •Don't necessarily represent etiological factors ,but they contribute a good deal to our understanding of the natural history of disease . Age: strongly related to disease than any other single host factor . Certain diseases –more frequent in certain age groups than in others –
  • 33. eg.measles in childhood cancer in middle age  atheroslerosisin old age As age advances –many chronic progressive disorders increse in prevalence. Bimodality Ther may be two separate peaks instead of one in the age incidence curve of a disease . eg leukemia,Hodgkin's disease.
  • 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 obesity Uncommon ’lung cancer,chd 1. Basic biological differences between the sexes including sex linked genetic inheritance 2. Cultural and behavioral differences between the sexes (alcoholism,automobile use etc.)due to different roles www.drjayeshpatidar.blogspot.in
  • 35. •ethnicity Differences in disease occurrence observed between population subgroupsof different racial and ethnic origin. • MARITAL STAUS Mortality rates were lower for males and females who are married. •Occupation •Social class •Upperer class •behavior Cigarette smoking  alcoholism Sedentary life style •Stress •Migration www.drjayeshpatidar.blogspot.in
  • 36. Measurement of disease Disease load –mortality,morbidity disability etc Mortality –straight forward Morbidity –2 aspects—incidence and prevalence Measurement can be extended to health statesand events. www.drjayeshpatidar.blogspot.in
  • 37. Comparing with known indices comparison between different populations and subgroups of he same population ,it is possible to arrive at clues to disease etiology . Formulation of a hypothesis Relating to disease etiology Should specify the following 1. The population 2. The specific cause being considered 3. The expected outcome 4. The dose response relationship 5. The time response relationship Eg. The smoking of 30-40cigaretes per day causes lung cancer in 10% of smokers after 20 years of exposure. www.drjayeshpatidar.blogspot.in
  • 38. Uses of descriptive epidemiology 1. Provide data regarding magnitude of the disease load and the type of diseases problems in he community in terms of morbidity and mortality rates and ratios 2. Provide clues to disease etiology and help in the formulation of an etiological hypothesis 3. Provide back ground data for planning ,organising and evaluating preventive and curative services. 4. they contribute to research by describing variations in disease occurrence by time ,place and person. www.drjayeshpatidar.blogspot.in
  • 39. Analytical epidemiology • The subject of interest is individual within the population • The object is to test hypothesis • 2 distinct types of obsnl. Studies 1. Case control studies 2. Cohort studies www.drjayeshpatidar.blogspot.in
  • 40. • From each of these one can determine • A. whether or not a statistical association exists between a disease and a suspected factor • If one exists –the strength of association www.drjayeshpatidar.blogspot.in
  • 41. Analytical epidemiology (Schematic diagram) Case control study indls with particular disease}cases Factors indls without particular disease}controls Present Or Absent PROSPECTIVE(cohort study) Indl. Exposed to particular factors. Inld unexposed to particular factors presence or absence of particular disease Time www.drjayeshpatidar.blogspot.in
  • 42. Case control study (retrospective study) 3 distinct features • Both exposure and outcome have occurred before the start of the study • It uses a control/comparison group • The study proceeds backwards from effect to cause. www.drjayeshpatidar.blogspot.in
  • 43. • Involves 2 populations • Cases controls www.drjayeshpatidar.blogspot.in
  • 44. basic steps • Selection of cases and control • Matching • Measurement of exposure • Analysis and interpretation www.drjayeshpatidar.blogspot.in
  • 45. Selection of cases and control 1. Selection of cases: a. Definition of case-involves 2 specifications— i. Diagnostic criteria: criteria of the disease and stage of the disease if any ii. Eligibility criteria :only newly diagnosed cases within a specified period of time are eligible www.drjayeshpatidar.blogspot.in
  • 46. b.Sources of cases • Hospitals :from a single or network of hospitals admitted during a specified period of time. • general population: all cases of study diseases occurring within a defined geographic area during a specified period of time are ascertained—often through a survey or ,a disease registry or hospital network. www.drjayeshpatidar.blogspot.in
  • 47. Selection of controls • Controls must be free from the disease under study. • They must be similar to the cases as possible ,except for the absence of the disease under study www.drjayeshpatidar.blogspot.in
  • 48. Sources of controls • Hospitals • Relatives • Neighbors • general population www.drjayeshpatidar.blogspot.in
  • 49. Matching • Comparability between cases and controls to be ensured this is done through - Matching • Defined as the process by which we select controls in such a way that they are similar to cases with regard to certain pertinent selected variables. which are known to influence the outcome of the disease and which if not adequately matched for comparability ,could distort or confound the results. www.drjayeshpatidar.blogspot.in
  • 50. Matching • A confounding factor is defined as the one associated with both exposure and disease, and is distributed unequally in the study and control groups. • Eg:Role of alcohol –esophageal cancer Smoking is a compounding factor---it is associated with the consumption of alcohol and it is an independent risk factor for esophageal cancer. in this case the effect of alcohol consumption can be determined only if the influence of smoking is neutralized by matching www.drjayeshpatidar.blogspot.in
  • 51. • Age could be compounding variable: Relationship between Steroid contraceptive and breast cancer Matching protects against an un expected strong association between the matching factor and the disease . www.drjayeshpatidar.blogspot.in
  • 52. Kinds of matching • Group • Pairs www.drjayeshpatidar.blogspot.in
  • 53. Measurement of exposure • Information about exposure should be obtained in precisely the same manner both for cases and controls • May be 0btained by interviews ,questionnaires or by studying past records of cases such as hospital records or employment cards www.drjayeshpatidar.blogspot.in
  • 54. Analysis To find out • Exposure rates among cases and controls to suspected factor • ESTIMATION OF DISEASE RISK ASSOCIATED WITH EXPOSURE (ODD RATIO) www.drjayeshpatidar.blogspot.in
  • 55. Exposure rates(FREQUENCY OF EXPOSURE) • Direct estimation of the exposure rates to suspected factor in disease and non disease group. www.drjayeshpatidar.blogspot.in
  • 56. A case control study of smoking and lung Ca Cases(with lung Ca) Controls (without lung Ca) Smokers <5 cigars/day 33(a) 55(b) Non smokers 2(c) 27(d) total 35(a+c) 82(b+d) Table 1 Exposure rates Cases=a/a=c=33/35=94.2% Controls=b/b+d=55/82=67%,p<0.001 Frequency rate of Lung Ca is higher among smokers than non smokers. If p</=0.o5----statistically significant www.drjayeshpatidar.blogspot.in
  • 57. Estimation of risk • Estimation of disease risk associated with exposure—is obtained by an index known as relative risk or risk ratio . Relative risk = incidence among exposed incidence among non exposed =a/(a+b) + c/(c+d) www.drjayeshpatidar.blogspot.in
  • 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 on 3asssumptions 1. The diseases being investigated must be relatively rare 2. The cases must be representative of those with the disease 3. Controls must be representative without disease www.drjayeshpatidar.blogspot.in
  • 59. Suspected or risk factors Case /disease present Control /disease absent present a b Absent Odds ratio=ad/bc c a+c d b+d Odds 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 that of nonsmokers. odd ratio is the key parameter in the analysis of case control studies www.drjayeshpatidar.blogspot.in
  • 60. Bias in case control studies • Any systematic error in the determination of the association between the exposure and disease • Reflects non comparability Bias—  confounding rectified by matching  memory or recall  selection—cases may not be representative in general population  Berkesonian –different rates of admission to hospitals for people with different diseases  Interviewer-double blinding is used to rectify www.drjayeshpatidar.blogspot.in
  • 61. Advantages &disadvantages Advantages • 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 minimal Disadvantage •Bias-on memory or past records, accuracy may be uncertain •Selection of appropriate control group difficult •Cannot measure incidence ,can only measure relative risk •Don’t distinguish between cause and associated factors •Concern of representative ness of cases and controls www.drjayeshpatidar.blogspot.in
  • 62. Cohortstudies(Longitudinal,prospective,incidence, forward- looking study) • Usually undertaken to obtain additional evidence to refute to or support of the existence of an association between suspected causes and diseases Features 1. Cohorts are identified prior to the appearance of the disease under investigation 2. Study groups ,so defined ,are observed over a period of time to determine the frequency of disease among them 3. Study proceeds forward from cause to effect www.drjayeshpatidar.blogspot.in
  • 63. Concept • Cohort –a group of people who share a common characteristic or experience within a defined time period Indications for cohort study • When there is a good evidence of an association between exposure and disease ,as derived from clinical observations and supported by descriptive and case control studies • When exposure is rare ,but the incidence of the disease high among exposed. • When attrition of study population can be minimized. • Ample funds are available www.drjayeshpatidar.blogspot.in
  • 64. framework • Cause to effect =exposure has occurred but not the disease cohort Disease Yes no total Exposed to putative etiologic factor a b A+b(study cohort) not Exposed to putative etiologic factor c d C+d(contr ol cohort) www.drjayeshpatidar.blogspot.in
  • 65. General considerations in assembling cohort • Free from disease • Both the groups equally susceptible • Both the groups should be comparable • Diagnostic eligibility criteria the disease must be defined before hand A well designed cohort study is considered the most reliable means of showing an association between a suspected risk factor and subsequent disease . www.drjayeshpatidar.blogspot.in
  • 66. types • Prospective—the out come has not yet occurred at the time of inv. begin in the present and continue in the future • Retrospective. outcomes have occuredbefore the inv.goes back in time • A combination of both: cohort from past recordsand assessed of dat efor outcome www.drjayeshpatidar.blogspot.in
  • 67. Elements • Selection of study subjects • Obtaining data on exposure • Selection of comparison of groups • Follow up • analysis www.drjayeshpatidar.blogspot.in
  • 68. Selection of study subjects • General population • Special groups These may be special groups or exposure groups that can readily be studied 1.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 to follow up for a protracted period. 2.Exposure groups: if the exposure is rare more economical procedure to select a cohort of persons known to have experienced the exposure.eg.workers in industries. www.drjayeshpatidar.blogspot.in
  • 69. Obtaining data on exposure Information can be obtained from 1. Cohort members:interviews,mail questionnaires 2. Review of records: certain kind of information can be obtained from records.eg.dose of radiation, kinds of surgery,detailas of medication 3. Medical examination or special tests: eg. BP,ECG. 4.Environmental surveys: obtaining information on exposure levels of the suspected factor in the environment where the cohort lived or worked. Information to be collected in a manner that will allow classification of cohort a. According to whether or not they have been exposed to the suspected factor or not. b.According to the degree of exposure In addition demographic variables also to be collected. www.drjayeshpatidar.blogspot.in
  • 70. Selection of comparison groups 1. Internal comparisons: no outside comparison groups. in built comparison groups. ie.single cohort enters the study ,and its members may ,on the basis of information obtained, be classified into several comparison groups according to the level of exposure to risk before the development of the disease in question.eg.smoking ,blood pressure, serum cholesterol The groups so defined, are compared in terms of their subsequent morbidity and mortality rates. www.drjayeshpatidar.blogspot.in
  • 71. Age standardized death rates /100000men per year by amount of current smoking Classification of exposure (cigarettes) No of deaths Death rate ½ pack 24 95.2 1/2pack-1 pack 84 107.8 1-2 pack 90 229.2 2packs+ 97 264.2 Mortality from lung Ca increases with increased no of cigarettes smoked –there is valid association www.drjayeshpatidar.blogspot.in
  • 72. b.External comparisons • External control Is used when information on degree on exposure is not available – evaluate the experience of the exposed group. Eg .smokers and non smokers ,a cohort of radiologist with a cohort of ophthalmologist. study and control variables should be similar in demographic and possibly important variables www.drjayeshpatidar.blogspot.in
  • 73. c.Comparison group with general population values • None available –mortality experience of the exposed group is compared with the mortality experience of the general population • Eg. Comparison of frequency of lung ca.among uranium workers with lung ca.mortality with general population where the miners resided. www.drjayeshpatidar.blogspot.in
  • 74. d.Follow up • Regular follow up is part of cohort studies. Procedure to obtain data: 1. Periodic medical examination of each member of the cohort 2. Reviewing physician and hospital records 3. Routine surveillance of death records 4. Mailed questionnaire ,telephone call s,periodic home visits-preferably all 3 on annual basis www.drjayeshpatidar.blogspot.in
  • 75. analysis The data are analysed in terms of 1. Incidence rates of outcome among exposed and non exposed 2. Estimation of risk www.drjayeshpatidar.blogspot.in
  • 76. Incidence rates • Can be calculated directly Cigarette smoking Developed lung ca Did not develop lung ca total yes 70(a) 6930(b) 7000(a+b) no 3(c) 2997(d) 3000(c+d) Incidence rates a. among smokers =70/7000=10/1000 b. Among non smokers=3/3000=1/1000 p<0.001 Contingency table applied to hypothetical cigarette smoking and lung cancer eg. www.drjayeshpatidar.blogspot.in
  • 77. Estimation of risks Risk of outcome : • relative risk • attributable ratio relative risk(RR)= incidence of disease /deathamong exposed incidence disease/death among nonexposed RR=10/1=10(data from table) relative risk (RR) is a direct measure or (index) of the strength of the association between the cause and effect RR-1-indicates no association >1suggests +association between exposure and the disease under study 2-indicates-incidence rate of disease is 2 times higher in ht exposed group as compared to un exposed Smokers are 10 times at greater risk of developing lung ca than non smokers Larger RR---greater the strength of association www.drjayeshpatidar.blogspot.in
  • 78. attributable risk (AR)(risk difference) • Difference in incidence rate of disease or death between an exposed and non exposed group • Expressed in % • AR= incidence of disease rate among exposed-incidence disease rate among nonexposed incidence of disease rate among exposed AR=10-1/10x100=90%(table data) Indicates to what extend the disease under study can be attributed to the exposure. www.drjayeshpatidar.blogspot.in
  • 79. Population of attributable risk • Incidence of the disease or death in the total population - Incidence of the disease or death among those who were not exposed to the suspected causal factor Deaths per 100, 000-years Heavy smokers 224 Exposed to suspected factor(a) Non smokers 10 Non exposed to suspected factors(b) Deaths in total population 74(c) Individual RR=a/b=224/10=22.4 Population AR=c-b/cx100=86% Deaths per 100, 000-years Heavy smokers 224 Exposed to suspected factor(a) Non smokers 10 Non exposed to suspected factors(b) Deaths in total population 74(c) Individual RR=a/b=224/10=22.4 Population AR=c-b/cx100=86%www.drjayeshpatidar.blogspot.in
  • 80. • The concept of population attributable risk is useful in that it provides an estimate of the amount by which the disease could be reduced in that population if the suspected factor was eliminated or modified • In the eg.--86%deaths could have been avoided if the risk of cigarettes were eliminated. www.drjayeshpatidar.blogspot.in
  • 81. Relative risk and attributable risk • RR is imp. in etiological enquiries. • its size is a better index for assessing the etiological role of a factor in disease. • The larger the RR ,the stronger the assocn.between cause and effect. • RR does not reflect the potential public health importance as does the attributable risk • AR gives a better idea than does RR of the impact of successful preventive or public health programme might have in reducing the problemwww.drjayeshpatidar.blogspot.in
  • 82. Cardiovascular risk 100,000patient years Ages 30-39 40-44 RR 2.8 2.8 AR 3.5 20.0 The RR and AR of cardiovascular complications in women taking oral contraceptives RR independent of age AR is .5times higher in the older age. This epidemiological observation is the basis for not recommending OCP to those aged 35yers and above www.drjayeshpatidar.blogspot.in
  • 83. Risk assessment smokers vs non smokers Cause of death Death rate/1000 Smokers nonsmokers RR AR Lung Ca .90 0.7 12.86 92.2 CHD 4.87 4.22 1.15 13. 3 Smoking is attributable to 92%of Calung&13.3%of CHD . In CHD,both RR and AR not very high –suggests not much of the disease could be prevented as compared to lung ca www.drjayeshpatidar.blogspot.in
  • 84. Advantages &disadvantages 1. Incidence can be calculated 2. Several possible outcomes related to exposure can be studied simultaneously 3. Provide direct estimate of RR 4. Dose response ratio –can be calculated 5. Bias can be minimized-groups formed before disease develops 1. Long time 2. Large no of people 3. Administrative problems-loss of experienced staff ,loss of funding and extensive record keeping are in evitable 4. Original cohort may be lo 5. Selection of controls limiting factor 6. There may be change in diagnostic criteria 7. Expensive 8. May alter peoples behavior 9. Limited no of factors are concentrated www.drjayeshpatidar.blogspot.in
  • 85. Main difference between case control &cohort 1. Proceeds from effect to cause 2. Starts with disease 3. Tests whether the disease occurs more frequently in those with disease than among those without disease 4. First Approach to testing hypothesis 5. Involve fewer no of subjects 6. quick results 7. Suitable for the study of rare disease 8. Only estimate of RR 9. Cannot yield information about disease other than that selected for study 10. Relatively in expensive 1. cause to effect 2. People 3. Test in those exposed 4. Reserved for testing precisely formulated hypothesis 5. Larger no of subjects 6. Long follow up period 7. Inappropriate for rare disease 8. Yields incidence rate,RR&AR 9. Can yield information about more than one disease outcome 10. expensive www.drjayeshpatidar.blogspot.in
  • 86. Experimental epidemiology • Equated with randomised controls • It is similar to cohort study except The conditions under which study is carried out is under the direct control of the investigator • Involve action, intervention or manipulation –deliberate application or withdrawal of the suspected causes or changing one variable in the causative chain in the experimental group while making no change in the control group. • Observing and comparing the outcome of the experiment in both ht egroups www.drjayeshpatidar.blogspot.in
  • 87. Aims 1. To provide scientific proof of etiologic factor which may permit the modification or control of those diseases 2. To provide a method of measuring the effectiveness and efficiency of health services for he prevention ,control and treatment of disease and improve the health of the community They may be conducted in animals and human beings www.drjayeshpatidar.blogspot.in
  • 88. Animal studies-application 1. Experimental reproduction of human disease in animals to confirm etiological hypotheses and to study their pathogenetic phenomena or mechanisms 2. Testing the efficacy-preventive and therapeutic measures such as vaccines and drugs 3. Completing a natural history of disease. www.drjayeshpatidar.blogspot.in
  • 89. Animal experiments Advantages • Can be bred in labs and manipulated easily • They multiply rapidly and genetic studies can be carried out • Disadvantages • Not all human diseases can be reproduced in animals • conclusions may not be strictly applicable to human beings www.drjayeshpatidar.blogspot.in
  • 90. human experiments • Will always have to investigate disease etiology and to evaluate the preventive and therapeutic measures. • Even more in essential in the investigation of diseases that cannot be reproduced in animals • The benefits of the experiment have to be weighed against risk involved. WHO has laid down strict code of practice in connection with human trials www.drjayeshpatidar.blogspot.in
  • 91. types • Randomised controlled trials-those involving a process of random allocation • Non randomised or non experimental trials www.drjayeshpatidar.blogspot.in
  • 92. Randomised controlled trials design Select suitable population(ref.or target) Select suitable sample Make necessary exclusions randomise Experiment group Control Manipulation and follow up Those not eligible Do not wish to give consent Assessment www.drjayeshpatidar.blogspot.in
  • 93. Steps in conducting RCT • Developing a protocol • Selecting reference and experimental populations • Randomisation • Manipulation and intervention • Follow up • Assessment of outcome www.drjayeshpatidar.blogspot.in
  • 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 and control groups treatment to be applied Protocol to be strictly adhered till end ,helps in preventing bias and source of error in the study Pilot/preliminary studies are done prior to protocol – feasibility/operational efficiency of certain procedures, or unknown effects ,or on the acceptability of certain policies www.drjayeshpatidar.blogspot.in
  • 95. Selecting reference and experimental populations 1. Reference or target population  The popln.to which the findings of the trial ,if found successful are expected to be applicable  Mankind/geographically limited/person in specific age/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 consent Representative of the population to which they belong Qualified or eligible for the trial www.drjayeshpatidar.blogspot.in
  • 96. 3.Randomisation  Statistical procedure Control study • Helps in removing bias and allow for comparability • Heart of control trial www.drjayeshpatidar.blogspot.in
  • 97. Manipulation • Deliberate application or withdrawal or reduction of the suspected causal factor as laid down in protocol • Creates independentvariable(drug,vaccine dietary component ,a habit etc.) whose effect is determined by measurement of the final outcome –dependent variable (incidence of disease, survival time, recovery period) www.drjayeshpatidar.blogspot.in
  • 98. Follow up • Examination of experimental and control group at defined intervals time, in a standard manner ,with equal intensity ,under the same given circumstances in the same time frame till final assessment of outcome • There can be attrition www.drjayeshpatidar.blogspot.in
  • 99. Assessment • Positive results :benefits of experimental measure • Negative results severity and frequency of side effects and complications Bias 1. Part of participants –subject variation 2. Observer bias- 3. Evaluation to rectify blinding is used. www.drjayeshpatidar.blogspot.in
  • 100. Study designs -controlled • Concurrent parallel :comparisons are made between 2 randomly assigned groups one exposed to treatment other not • Cross over type :each pt.serves as his own control Randomised Clinical trials Preventive trials Risk factor Cessation experiments Trial of etiological agents Evaluation of health services www.drjayeshpatidar.blogspot.in
  • 101. Nonrandomized trials • Uncontrolled trials • Natural experiments • Before and after comparison studies www.drjayeshpatidar.blogspot.in

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