Methods of Epidemiology.pptx community health nursing

this article dives a nutshell of methods of epidemiology.

1. Methods of Epidemiology

2. Observational studies allow nature to take its own course, the investigator measures but does not intervene. Descriptive study limited to a description of the occurrence of a disease in a population. An analytical study goes further by analyzing relationship between health status and other variables. Experimental (or) intervention studies involve an active attempt to change a disease determinant (or) the progress of a disease.

3. DESCRIPTIVE EPIDEMIOLOGY Are usually 1 st phase of an epidemiological investigation. Are concerned with observing the distribution of disease (or) health related characteristics with which the disease in question seems to be associated.

4. Procedures in Descriptive Studies Defining the population to be studied. Defining the disease under study. Describing the disease under study. Measurement of disease. Comparing with known indices. Formulation of an aetiological hypothesis.

5. 1. Defining the population Descriptive studies are investigations of populations. The 1 st step is to define the “population base” In terms of total number, composition by age, sex, occupation, cultural characters etc ,. The defined population can be the whole population in a geographic area.

6. The defined population can also be specially selected group such as age and sex groups, occupational groups, hospital patients, school children, small communities. It needs to be large enough. The community chosen should be stable, without migration into (or) out of the area. It should be clear who does and who does not belong to the population.

7. 2. Defining the Disease under the study Once the population to be studied is defined or specified, one must define the disease or condition being investigated. The definition should be both precise and valid to enable him to identify those who have the disease from those who do not.

8. The diagnostic studies must be acceptable to the population to be studied and applicable to their use in large populations.

9. 3. Describing the disease The primary objective of descriptive epidemiology is to describe the occurrence and distribution of disease by time, place and person, and identifying those characteristics with presence or absence of disease in individuals.

10. a) Time distribution The pattern of disease may be described by the time of its occurrence, ie,. by week, month, year, our of onset etc,. i. short term fluctuations ii.long term fluctuations iii. Long term or secular trends.

11. b) Place distribution Studies of the geography of the disease Geographic pattern provide an important source of clues about the causes of the disease. i . international variations ii. National variations. iii. Rural urban variations. iv. Local distributions.

12. c)Person distribution The disease is further characterized by age, sex, occupation, marital status, habits, social class and other host factors.

13. 4. Measurement of disease Clear picture of the amount of disease (disease load) in the population. By morbidity, mortality, disability and so on. Morbidity has 2 aspects- incidence and prevalence.

14. Incidence can be obtained from ‘longitudinal studies’ Prevalence from ‘cross sectional studies’

15. Cross sectional studies Simplest form of an observational studies. It is based on a single examination of a cross section population at one point in time. Also known as “prevalence study” Also more useful for chronic diseases.

16. Longitudinal studies Are useful To study the natural history of disease and its future outcome. For identifying risk factors of disease. For finding out incidence rate Are difficult to organize and more time consuming than cross sectional studies.

17. 5. Comparing with known indices The essence of epidemiology is to make comparisons and ask questions. By making comparisons, it is possible to arrive at clues to disease etiology.

18. 6. Formulation of Hypothesis A hypothesis is a supposition, arrived at from observation or relation. It can be accepted or rejected, using the techniques of analytical epidemiology.

19. Uses of Descriptive Epidemiology Provide data regarding the magnitude of disease and types of disease problems in the community. Provide clues to disease aetiology and help in the formulation of an etiological hypothesis. Provide background data for planning, organizing and evaluating preventive and curative services.

20. They contribute to research by describing variations in disease occurrence by time, place and person.

21. ANALYTICAL EPIDEMIOLOGY They are the second major type of epidemiological studies. Descriptive studies – entire populations Analytical studies – individual within the population. Objective – not to formulate but to test hypothesis.

22. Analytical studies - Types

23. Design of a case control study direction of enquiry Start with Population Cases Controls Exposed Not exposed Exposed Not exposed

24. CASE CONTROL STUDY Often called “Retrospective studies” 3 distinct features. Both exposure and outcome have occurred before the start of the study. The study proceeds backwards effect to cause. It uses a control (or) comparison group to support (or) refute an inference.

25. It involves 2 populations : cases and controls. The unit is the individual. The focus is on the disease (or) some other health problem that has already developed. They are basically comparison studies. Cases and controls must be comparable with “confounding factors” such as age, sex, occupation, social status etc,.

26. Basic steps: Selection of cases and controls Matching. Measurement of exposure. Analysis and interpretation.

27. 1. Selection of cases & controls 1. Selection of cases. Definition of case: i )Diagnostic criteria ii)Eligibility criteria b)Sources of case: i )Hospitals ii)General population

28. 2. Selection of Controls The controls must be free from the disease under the study. Sources of controls i)Hospital controls ii)Relatives iii)Neighbourhood populations iv)General population

29. 2. Matching Matching is done to ensure ‘comparability’ between cases and controls. A ‘confounding factor’ is defined as one which is associated with exposure and disease and is distributed unequally in study and control groups.

30. 3. Measurement of Disease Information about exposure should be obtained in the same manner both for cases and controls. By interviews, questionnaires by studying the hospital records, employment records etc,.

31. 4. Analysis To find out; Exposure rates among cases and controls to suspected factor. Estimation of disease risk associated with exposure (Odds ratio)

32. Exposure rates: Case control study gives direct estimation of the exposure rates to a suspected factor in cases (disease group) and controls (non-diseased groups). Cases (with lung cancer) Controls (without lung cancer) Total Smokers (less than 5 cigarettes per day) 33 (a) 55 (b) 88 ( a+b ) nonsmokers 2 © 27 (d) 29 ( c+d ) Total 35 ( a+c ) 82 ( b+d ) n= a+b+c+d

33. a= no of people who are exposed and have the outome b= no of people who are exposed and do nt have the outcome c= no who are nt exposed and have the outcome d=no who are not exposed and do not have the outcome a+b = total number who are exposed c+d = total no who are not exposed a+c = total no who have the outcome b+d = total no who do not have the outcome a+b+c+d = total study population

34. Therefore, exposure rate among cases = a/ a+c Exposure rates among controls = b/ b+c

35. Relative risk: Defined as the ratio between the risk of disease in the exposed group and the risk of disease in the non-exposed group. Relative risk (RR) = In case control studies, it is not possible to calculate the risk of disease because the population at risk is not known. RR can be obtained directly through Cohort studies. We calculate odds ratio in case control studies.  

36. Odds Ratio Assess the strength of an association between an exposure (risk factor) and the outcome of interest. Odds ratio (OR) = ad/ bc . a= persons with lung cancer (cases) who smoked b= controls who smoked c= persons with lung cancer(cases) who did not smoke d= controls who did not smoke

37. exposure Individuals with lung cancer (cases) Without lung cancer (controls) smokers 19 (a) 22 (b) nonsmokers 15 (c ) D (63) Odds ratio (OR) = ad/ bc . = = 3.63  

38. Bias in case control studies Bias due to confounding Memory or recall bias Selection bias Berkesonian bias Interviewer’s bias

39. Advantages Relatively easy to carry out. Rapid & inexpensive. Require comparatively few subjects. Suitable to investigate rare diseases or diseases about which little is known. No risk to subjects. Allows the study of several different aetiological factors.

40. Risk factors can be identified. No attrition problems, because case control studies do not require follow up of individuals. Ethical problems are minimal.

41. Disadvantages Problems of bias relies on memory or past records, the accuracy of which may be uncertain. Selection of an appropriate control group may be difficult. Cannot measure incidence, can only estimate the relative risk.

42. Do not distinguish between causes and associated factors. Not suited to the evaluation of therapy on prophylaxis of disease. Representativeness of cases and controls.

43. COHORT STUDY TIME DIRECTION OF ENQUIRY POPULATION PEOPLE WITHOUT DISEASE EXPOSED NOT EXPOSED DISEASE NO DISEASE DISEASE NO DISEASE

44. It is known by a variety of names: Prospective study Longitudinal study Incidence study Forward looking study

45. It is usually undertaken to obtain additional evidence to refute or support the existence of an association between suspected cause and disease.

46. Features of Cohort Study The cohorts are identified prior to the appearance of the disease under investigation. The study groups, so defined, are observed over a period of time to determine the frequency of disease among them. The study proceeds forward from cause to effect.

47. Cohort “a group of people who share a common characteristic or experience within a defined time period ( eg ; age, occupation, exposure to a drug or vaccine, pregnancy, etc ,.)

48. Types of Cohort Studies Prospective cohort studies (current; longitudinal study) Retrospective cohort studies (historical cohort study) A combination of retrospective and prospective cohort studies.

49. Elements of a Cohort study Selection of study subjects Obtaining data on exposure Selection of comparison groups Follow up Analysis

50. 1. Selection of study subjects General population Special groups Select groups Exposure groups

51. 2. Obtaining data on exposure Cohort members Review of records Medical examination or special tests. Environmental surveys

52. 3. Selection of comparison groups Internal comparisons External comparisons Comparison with general population rates

53. 4. Follow up One of the problems of cohort study is the regular follow up of all participants. Therefore, at the start of the study methods should be devised, to obtain the data for assessing the outcome. Periodic medical examination Review physician and hospital records Death records Mailed questionnaires, telephone calls, home visits.

54. 5. Analysis Incidence rates of outcome among exposed and non-exposed. Estimation of risk.

55. 1. Incidence rate; Eg : 300 new cases of a specific disease with the population of 60000 in the year 2013. what is the incidence rate? In the year 2013 = 300/60000 = 5 per year  

56. 2. Estimation of risk; Estimates the risk of outcome (disease or death) RR is the ratio of incidence among exposed to incidence among nonexposed . RR =  

57. The larger the RR, the greater the strength of association between the suspected factor and the disease. RR of 1 = no association RR of > 1 = positive association

58. Attributable risk AR is the difference in incidence rates of disease between an exposed and nonexposed group. Also known as “risk difference” AR indicates the no of cases of a disease among exposed individuals that can be attributed to that exposure.

59. AR = Incidence rate of lung cancer among exposed is 40 and 4 in non exposed. AR = Inference; 90% of lung cancer among smokers are due to smoking.  

60. Advantages of Cohort study Incidence can be calculated. Several possible outcomes related to exposure can be studied simultaneously. Cohort studies provide a direct estimate of relative risk.

61. Disadvantages Involve a large number of people. It takes a long time to complete the study and obtain the results. Administrative problems such as loss of experienced staff, loss of funding and expensive record keeping are inevitable.

62. Lose a substantial portion of cohort – they may migrate, lose interest in the study. Selection of comparison groups which are representative of the exposed and unexposed segments of the population is a limiting factor. There may be changes in the standard methods or diagnostic criteria of the disease.

63. Are expensive. The study itself may alter people’s behaviour. Ethical problems. We must concentrate on a limited number or factors possibly related to disease outcome.