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  1. 1. ANALYTICAL STUDY Dr Payal Resident doctor, IGGMC Nagpur
  2. 2. Contents •Introduction •Epidemiology •Methodology •Study design
  3. 3. Introduction • Epidemiology : ( comes from a Greek epidemic meaning upon the people) “Epi” –among, “demio”-population, “logy”- study. • The study of distribution and determinants of health related states or events in specified populations and the application of this study to the prevention and control of health problems.
  4. 4. Dr. JOHN.M.LAST
  5. 5.  Distribution: Time, Place, Person  Determinants: Causes or risk factors  Frequency: Incidence, Prevalence
  6. 6.  Distribution of disease:  Time- seasonal(e.g.URTI in winter) Place- geographical( Kala azar in UP,BIHAR,WB Person- age(measles in 6month to 3 years, RF in 5to 15 years, cataract in>55yrs) -sex (migrain, DM in females, RTA in males)
  7. 7. Epidemiological Purposes in Public Health Practice • Discover the agent, host and environmental factors that affect health. • Determine the relative importance of causes (risk factors) of illness, disability and death. • Evaluate the effectiveness of health programs and services in improving population health.
  8. 8. Classification of Epidemiological Studies Epidemiological Study Type Observational Experimental Descriptive Analytic
  9. 9. Epidemiological Studies 1) Observational studies : The investigator measures but do not intervene. Descriptive study : • It is a first step in an Epidemiological Studies. • Defining the population(Unit of study is entire population) • Defining the disease (Inclusion , exclusion and diagnostic criteria) • Description of the occurrence of a disease in a population in terms of time, place ,person.
  10. 10. • Measurements of disease. • Comparing with known indices. • This study helps to generate the hypothesis. (Hypothesis: It is a logical supposition, a reasonable guess. Assumption without proof)
  11. 11. Analytical study : • Second major type • This study analyses the relationship between health status and other variables. • Unit of study is individual. • This study helps to test the hypothesis.
  12. 12. 2) Experimental study : aka Intervention study • This study confirm the hypothesis. • It involves an active attempt to change a disease determinant such as an exposure or a behaviour or the progress of a disease through treatment. • These studies are similar in design to experiments in other sciences.
  13. 13. 1)CROSS SECTIONAL STUDY • Aka prevalence study. • It is simplest form of an observational study. • Unit of study is individual. • It is based on a single examination of cross section of population at one point in time so the results of which can be projected on whole population.
  14. 14. • In this study, measurements of exposure and effects(outcome) are made at the same time, so this study is useful for investigating the casual association. • Study is similar to a survey in such a way that, it provides a snapshot of the population at a point in time. So also called as snap chat study. --(Ruspini,2002)
  15. 15. • Using this study, the epidemiologist defines the target population, then collects data from the population or a subset of the population at one specific point in time. • Participants are selected regardless of their exposure or disease status. • This study useful in sudden outbreak of disease and for chronic diseases.
  16. 16. Cross sectional study continued…….. • For example ,in a study of prevalence of hypertension, we collect data during survey about age, gender, body weight, physical exercise, salt intake and other variables. • Study helps in assessing load of disease, needs, resources, patterns of health utilization, practices and knowledge attitudes. • This study tells us about the distribution of a disease in a population rather than its etiology.
  17. 17. Advantages • Relatively quick and easy to conduct, no follow up. • Data related to all variables is collected at once. • Study includes multiple outcomes and exposures. • It is a best way to understand disease load and plan health services. • Generation of hypothesis.
  18. 18. Disadvantages • It is difficult to find out whether the outcome followed exposure in time or it is resulted due to outcome. • Not suitable for rare disease or disease with shorter duration. • Study measures prevalent cases( new and old cases) rather than incident cases so we can’t measure incidence. • The associations obtained in this study would be difficult to interpret.
  19. 19. 2)CASE CONTROL STUDY • Aka case referent or retrospective study. • This is the first approach to test a casual hypothesis. • Study Disease > Risk factor ( Backward looking) as the Investigator is looking backwards from the disease to a possible cause. • In this study, subjects are selected on the basis of whether they do (cases) or do not (controls) have the disease. • The groups are compared with respect to the proportion having a history of an exposure .
  20. 20. Framework of case control study Suspected or risk factors(Smoking) Cases (Lung Cancer) Controls (No lung Cancer) Present a b Absent c d Total a+c b+d
  21. 21. Steps in Case Control Study 1) Selection of Case – A case is someone who has disease under investigation. • It should be objective in nature and should be followed meticulously throughout the study. • Cases should represent a specified population group on the basis of disease, not exposure. For this one needs diagnostic criteria of that particular disease and also stage of the disease • The Second criteria is related to eligibility. The old cases or cases that have advanced stage of the disease may not be eligible for inclusion in the study.
  22. 22. • Sources of cases : 1. Hospitals: From a single hospital or network of hospitals either case series or a random sample, admitted during a specified period of time. ( convenient) 2. General population: selected in a geographical area through survey, a disease registry or hospital network.
  23. 23. 2) Selection of control • Control is someone who doesn’t have the disease under investigation. • Control should be similar to the cases as possible except for the disease under study • So controls should be indentified before the study. • Sources: controls from Hospital, relatives , neighbors, occupational associates or general population..
  24. 24. 3) Matching • The controls may differ from the cases in a number of factors such as age, sex, occupation, education, social status etc. • To share the comparability between cases and controls matching is done. • It is the process, by which select controls in such a way that they are similar to cases with regard to certain pertinent variables, which are known to influence the outcome of the disease. • If Matching not done ,this could distort or confound the results.
  25. 25. • Confounding factor – Is defined as one which is associated with both exposure and disease and is distributed unequally in cases and control group. • Precautions while matching - matching factor should be associated with disease not on exposure
  26. 26. 4)Measurements of exposure • In this study, the exposure status of the cases is usually determined after the development of the disease and by Questionnaire, physical examination, laboratory investigations and study of records. • The exposure may be of various types.(e.g. habit of alcohol drinking, smoking) • The measurement of exposure can be done by various methods. Such as estimation of causative agent in environment (carbon monoxide in air), estimation of pathological changes(RBC stripling,Hb level in lead poisoning).
  27. 27. 5) Analysis • Final step in analysis, done to find out (a) Exposure rates among cases and controls to suspected factor : provides direct estimation of exposure rates(frequency of exposure) to a suspected factor in disease and non disease. (b)Elimination of disease risk associated with exposure.
  28. 28. Smoking Lung Cancer Not having Lung cancer Total Exposure present 90(a) 20(b) 110 Exposure absent 10(c) 80(d) 90 Total 100 100 200
  29. 29. • Exposure rates:  Cases = a /(a+c) = 90/100 Controls = b /(b+d) = 20/100 This shows that, frequency of lung cancer was higher among smokers than among non smokers.
  30. 30. Estimation of Risk • It is obtained by Relative Risk(RR) or risk ratio is defined as the ratio between the incidence of disease among exposed persons and incidence among non exposed. Incidence among exposed Relative risk = Incidence among nonexposed = a /(a+c) ÷ b /(b+d)
  31. 31. • A typical case control study, does not provide incidence rate from which relative risk can be calculated. • Next step Is to estimate the risk of disease associated with the exposure Odds ratio (Cross product ratio) : Determine the association of an exposure and a disease. • It is similar to risk ratio
  32. 32. Derivation of odds ratio is on 3 assumptions: • The disease under investigation must be relatively rare. • The cases must be representative of those with the disease • The controls must be free from disease.
  33. 33. • Odds ratio = ( a/ b) ( c/ d ) = ad / bc = 90* 80/ 20* 10 = 7200 / 200 = 36
  34. 34. • Interpretation: Odds ratio is 1 , no association Odds ratio is >1 , positive association Odds ratio is <1 , negative association (protective effect) In the above example, people who smoke(cases) were having a risk of developing lung cancer 36 times that of non smokers.
  35. 35. Bias • It is any systematic error in the determination of the association between exposure and the disease. a) Bias due to confounding b) Memory or recall bias c) Selection bias d) Berkesonian bias e) Interviewers bias
  36. 36. Advantages • Easy to conduct, consume less time, inexpensive. • Suitable for rare disease. • There is no risk to the subjects as disease has already occurred. • It is possible to study different causative factors responsible for a particular disease. • Minimal ethical problem
  37. 37. Disadvantages • Study depends upon on history/records/memory. • It is very difficult to get a perfect control. Group. • It is not possible to measure incidence and relative risk. • This study can not distinguish between causes and associated factors
  38. 38. 3) COHORT STUDY • Cohort studies also called as Follow up , Incidence study, Prospective study, Forward looking study or longitudinal study. • Cohort study is similar to a longitudinal descriptive study with exception that there is comparison group in cohort. • Cohort is defined as a group of people who share a common characteristics or exposure within a specified time period (Birth cohort).
  39. 39. • Cohort studies are indicated when there is good evidence of association between exposure and when exposure is rare and incidence is high. • They are indicated when follow up is easy, cohort is stable, cooperative and easily accessible and ample funds are available. • Cohorts must be free from the disease under study and then they are classified into subgroups according to exposure to a potential cause of disease or outcome. And followed up for study under same identical conditions.
  40. 40. • Both groups should be comparable in respect of all the possible variables, which may influence the frequency of disease. • Diagnostic and eligibility criteria of disease must be defined before the study.
  41. 41. Types of cohort • 1.Prospective • 2.Retrospective • 3.Mixed
  42. 42. Steps of a cohort study 1) Selection of study subjects a) General Population: people residing in well defined geographical area. b) Special groups: such as professional group, governments employees, volunteers, etc. these groups are usually homogeneous population.
  43. 43. 2) Obtaining data on exposure: data collected through personal interviews, mailed questionnaires, previous records, medical examination or special tests. • Groups are divided according to whether they have been exposed to a suspected factor (case) or not exposed (control) if yes then divided again as per degree of exposure
  44. 44. 3) Selection of comparison group: Internal comparison: In some study no outside comparison group required. On the basis of information obtained single cohort group are classified into several comparison groups according to degree or level of exposure to risk before development of disease.(e.g. smoking, blood pressure) External comparison: when degree or level of exposure is not availble.
  45. 45. Comparison with general population: If none available or the difficulties of selecting the study and comparison group.
  46. 46. 4) Follow up: It is a most important step. To obtain data for assessing the outcome, the procedures required, A. Periodical medical examination of each member of cohort as it provides more information. B. Reviewing records from physicians and hospitals. C. By surveillance of routine death records. D. Mailed questionnaire, telephone calls and periodic home visits on annual basis.
  47. 47. 5) Analysis: In this study, we analysis data by calculating incidence rates of disease for both cohorts, and then estimating relative risk, attributable risk and population attributable risk.
  48. 48. Disease present Disease absent Total Cause present a (30) b (70) a+b (100) Cause absent c (10) d (90) c+d (100)
  49. 49. 1. Relative Risk: It is defined as the ratio between the incidence of disease among exposed persons and incidence among non exposed. • In above example, Incidence rate among smokers is=30/100 Incidence rate among non smokers is=10/100
  50. 50. • Therefore, Relative risk = 30/10 = 3. • Relative risk has etiological enquiries as it provides direct measure of the strength of association between suspected cause or exposure and effect. • In this example, relative risk is 3 means people who smoke cigarettes, have 3 times more risk of developing lung cancers than non smokers.
  51. 51. 2. Attributable risk/Risk difference: It is the difference in incidence rates of disease between exposed group and non exposed group. • It provides info. about the contribution of suspected cause for the occurrence of disease. (or how much of the disease can be attributed to suspected cause.)
  52. 52. AR = (Incidence of disease in exposed – Incidence of disease in non exposed × 100 Incidence rate among exposed
  53. 53. • In this example, AR= 0.3-0.1 × 100 0.3 = 0.2/0.1 × 100 = 66% • So in this example, It shows that, 66% of the lung cancers among smokers could be attributed to smoking. And hence there is a causal association.
  54. 54. • This information helps in knowing how much of the disease can be eliminated by eliminating or controlling the suspected cause under study.
  55. 55. 3. Population attributable risk: It is the incidence of the disease (or death) in the total population minus incidence of the disease (or death) among those who were not exposed to the suspected causal factor. • It provides an estimate of the amount by which the disease could be reduced in that population if the suspected cause or exposure was eliminated or controlled.
  56. 56. Advantages • We find out incidence. • The beginning point is cohort with and without exposure, so from this we can study many outcomes related to exposure. • Relative risk is being calculated. • Minimal bias
  57. 57. Disadvantages • Unsuitable for rare diseases. • Time consuming. (migration, death ,behaviour change or loss of interest, population attrition) • Administrative problems( lack of experienced staff, shortage of fund). • Over a period of time, the diagnostic criteria or definition of a disease under study may change. • Expensive.
  58. 58. • Selection of comparison groups which are representative of the exposed and unexposed segments of the population is a limiting factor. • Ethical issues.
  59. 59. Nested case control study • It is a case control study “nested” within a prospective or retrospective cohort study. • In this study design, Cases and Controls both are from cohort sample. • It is useful for predictor variables that are expensive, useful for costly analysis of specimens, like biochemical analysis of serum samples that are taken at the beginning of the study and then preserved for later analysis.
  60. 60. • STEPS : • Identify a cohort with a pool of specimens collected at baseline. • Identify those developing disease during follow up(Cases). • Select the sample from cohort who do not develop the disease(Control) • Measure the predictor variables.
  61. 61. 4)Ecological study • Aka correlational study • Useful for generating hypothesis • Units of analysis are groups of people • These studies can be done by comparing populations in different places at same time or in a time series or comparing same population at one place at different time.
  62. 62. Drawbacks • As it is easy to conduct, attractive, these study are often difficult to interpret. • Ecological studies usually rely on data collected for other purposes. • The association between exposure and effect at the individual level cant be made. • Ecological fallacy: bias results from if inappropriate conclusions are drawn on the basis of ecological data.
  63. 63. References
  64. 64. Study Design Flow Chart
  65. 65. THANK YOU

Notas del editor

  • Definition: by Dr. John.M.Last
  • Surveillance: The ongoing systemic collection, analysis and interpretation of health data essential to the planning, implementation and evaluation of public health practice,closely integrated with the timely dissemination of these data to those who need to know. [ centers for disease control and prevention proposed by Langmuir 1963]
  • Hypothesis: . It provides a tentative explanation for a phenomenon under investigation
  • that are fixed characteristics of individuals such as ethnicity or blood group. (The study involves looking at people who differ on one key characteristic at one specific point in time.)
  • Case Control studies are longitudinal.
  • Data : will be collected from hospital ,workplace or general population
  • Pertinent – getting attached/affiliated
  • easily accessible which minimizes the problem of attrition of study population.
  • Term prospective refers to the timing of data collection