Descriptive Epidemiology & Study design Potjaman Siriarayapon Bureau of Epidemiology

2 Outline  General concept of descriptive epidemiology  Study design in epidemiology Descriptive Analytic

3 Banluang district, Nan province

4 Average Rate of Cholangiocarcinoma among in- patients visiting Nan Hospital, by district, Nan province, Source : ICD10 Nan Hospital

5 Average Rate of Hepatocellular carcinoma among in-patients visiting Nan Hospital, by district, Nan province, Source : ICD10 Nan Hospital

6 How would to do to answer this problem?

7 DEFINITION OF EPIDEMIOLOGY "Epidemiolgy is the study of the distribution and determinants of health- related states or events in specified populations and the application of this study to the control of health problems" (John M. Last, 1988)

8 KEY CONCEPTS “Descriptive epidemiology” “Descriptive epidemiology” is the first step in epidemiological study. Careful observation of available information alone have led many successful preventive measures in the past. Describing health or disease occurrence according to place, person and time can lead to very meaningful hypothesis formulations.

9 TIME  Disease rates change over time. Some of these change occur regularly and can be predicted.  By examining events that precede a disease rate increase or decrease, we may identify causes and appropriate actions to control or prevent further occurrence of the disease.

10 Reported Cases of Leptospirosis per 100,000 Population, Thailand, Secular (long-term) trends: Secular (long-term) trends: to predict or evaluate control program Influx of rat

11 Seasonality: to see seasonal pattern over years Cases of eosinophilic meningitis, Nan province, Thailand, Cases Onset by Month

12 Day of week and time of day: especially important for condition that related to occupational or environmental condition Deaths Time of Day Fatalities associated with farm tractor injuries by time of day, Georgia,

13 Epidemic period: to show the time course of disease outbreak Flood Cases Date of onset Number of leptospirosis patients by date of onset, Nov 25-Dec 21, 2000

14 PLACE  We describe a health event by place to gain insight into the geographical extent of the problem.  We may use place of residence, birthplace, place of employment, school district, hospital unit, etc., depending on which may be related to the occurrence of the health event.

Myanmar Umphang district Nov.-Dec. Jan-Feb. Mar. village Refugee camp Meningococcal cases, Tak province,

16 Thai-Myanmar border area in Umphang district

17 In the refugee camp

18 House of Karen people outside the camp

19 PERSON  There are several person categories available: inherent characteristics: age, sex, race acquired characteristics: marital status, immune activities: occupation, use of medication/tobacco/drugs condition under which their live: socioeconomic status, access to medical care

20 Sex specific attack rates of reported leptospirosis cases, Songkla, Nov 25-Dec 15, 2000

21 Age specific attack rates of reported leptospirosis cases, Songkla, Nov 25-Dec 15, 2000

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23 Study design

24 Why “type of study”? Design more appropriate studies to get right answers to the specific questions Getting more optional study design to the questions Shape up proposals for getting more valid results

TYPES OF OBSERVATIONAL STUDIES Descriptive Studies Little is known about the occurrence, or determinants of the disease - Identify cases, estimate disease frequency, examine time trend - Justify additional (analytic) studies to test hypotheses Analytic Studies Enough is known about the ddisease, specific hypotheses can be tested - Test specific etiologic hypotheses - Justify additional studies - Suggest potential for disease prevention

26 STUDY DESIGN OF DESCRIPTIVE STUDY

27 STUDY DESIGN OF DESCRIPTIVE STUDY  Individual level Case reports or case series Cross-sectional surveys of individual  Population level Ecological study (Correlational study)

28 CASE REPORTS AND CASE SERIES  Case reports :  Case reports : describe experience of a single patients. Case reports document unusual medical occurrence and can represent the first clues in the identification of new diseases.  Case series :  Case series : collections of individual case reports. Investigation of the activities of the individual in case reports can lead to formulation of a hypothesis.

29 SARS situation

30 CASE REPORTS AND CASE SERIES  Stength discover new diseases bring background information to form hypothesis about risk factors  Weakness might base on only one case do not have a comparison group

31 CROSS-SECTIONAL STUDY Total population Cases

32 Character of cross-sectional study  Collect data for all population; case & healthy people  Information obtain is the number of total cases at that study period: prevalence  Can be either descriptive or analytic, depend on design

33 CROSS-SECTIONAL STUDY Descriptive  Collected number of cases and number of total population  Can assess only prevalence of disease or other health events, also called prevalence study Analytic  Exposure and disease status are assessed simultaneously  Can determine association between exposure and disease

34 Vaccine coverage among Thai children  Proportion of children received vaccine  Education level of mother and vaccine status in children

35 Cross-sectional study Defined Population Exposed Have disease Not exposed: Have disease Not Exposed : Do not have disease Exposed Do not have disease Gather Data on Exposure and Disease

36 Hypothetical illustration of the interrelationship between an occupational exposure and prevalence of disease Job A (hazardous) 100Workers 80 well 20 ill Job B (non-hazardous) 100 Workers 95 well 5 ill 80 well 10 ill 95 well 15 ill 10 change jobs due to illness Point X Prevalence of job A= 20 % ( 20/100 ) Prevalence of job B= 5 % ( 5/100 ) Prevalence ratio = 4

37 Hypothetical illustration of the interrelationship between an occupational exposure and prevalence of disease Job A (hazardous) 100Workers 80 well 20 ill Job B (non-hazardous) 100 Workers 95 well 5 ill 80 well 10 ill 95 well 15 ill 10 change jobs due to illness Point Y Prevalence of job A= 11 % ( 10/90 ) Prevalence of job B= 14 % ( 15/110 ) Prevalence ratio= 0.8

38  Difficult to establish temporal relationship  Can assess only association but not a “causal association”  Length biased sampling: Disease that has long duration will over-represent the magnitude of illness while short duration will under-represent illness. PROBLEM OF CROSS-SECTIONAL STUDY

39 STUDY DESIGN OF DESCRIPTIVE STUDY  Individual level Case reports or case series Cross-sectional surveys of individual  Population level Ecological study (Correlational study)

40 ECOLOGICAL STUDY  These studies are an extension of the use of routine data.  Essentially the average exposure of the population is plotted against the rate of the outcome for that population  This is done for several populations and the data are then examined for evidence of an association between exposure and outcome.

Group of Proportion of Suicide rate Prussian Province Protestant (per 100,000 pop.) A B C D Study to find relationship between suicide rate and proportion of Protestant

42 Prov. Proportion of Suicide Protestant rate A B C D Suicide rate (per 10 5 /year) Proportion of Protestant Correlation between proportion of protestant and suicidal rate in 4 groups of Prussian provinces

43 Rationale for ecological study  Low cost and convenience: make use of existing data  Measurement limitations of individual- level studies: environmental study  Interest in ecological effect: social norm, population intervention

44 ECOLOGICAL STUDY Strength:  Strength: cheap, quick and convenient since it usually come from existing data Weakness:  Weakness: inability to link exposure with disease in individual (ecological fallacy) limit to control effect of other factors

45 In summary Ecological study is a convenient, cheap and simple study. Unit of study is the aggregate data not individual level. It is usually be conducted as the first step study for research. The result is difficult to interpret because of confounding and bias. Ecological or correlation study

46 Summary of the study design

Analytic study

Disease among exposed? Disease among non-exposed? Usuallyprospective Usually prospective Cohort study Population at risk Exposed Not Exposed and

49 Distribution of illness according to exposure in a cohort study Exposed Not exposed Ill Not ill a b c d a+b c+dRisk a+b c+d a c Relative risk = Risk exposed / Risk not exposed

50 Key features  Should have follow up period (even in retro-cohort)  Have confine population  Comparison group should be as similar as possible

51 Cohort study Advantage Advantage  Determine multiple effect of single exposure  Able to estimate incidence  Minimize bias for prospective study Disadvantage Disadvantage  Expensive and time consumed  Validity of result depend on follow up

Disease (Case) Exposure ? Case-control study No disease (Control) ?

53 Distribution of cases and controls according to exposure in a case-control study Exposed Not exposed Total Cases a c a+c Odds of exposure a/c Controls b d b+d b/d b/d Odds ratio = Odds E (cases) / Odds E (controls)

Odds  Team A plays 10 games: 8 wins 2 defeats  Risk (probability) of losing = 2/10  Odds of losing = 2/10 = 2/8 8/10 Probability of event Probability of non-event

Key features  Study only some part of the population, especially among non-case  Case and control should come from the same source population, to allow possibility that control could receive the exposure  Case and control are not necessary to be the same

56 Advantage Advantage  Cheap, quick and efficient for rare disease  Potential role for testing multiple risk factors Disadvantage Disadvantage  Susceptible to bias  Inefficient for rare exposure  Sometime difficult to establish temporal relationship between exposure and disease Case-control study

57 Advantage: Feasible and practical especially for ethical concern Feasible and practical especially for ethical concern Study population is more represent of the target population Study population is more represent of the target populationLimitation: Difficult to replicate Difficult to replicate Less control of extraneous factors that will lead to distortion of the result Less control of extraneous factors that will lead to distortion of the result Less secure to make generalization Less secure to make generalization OBSERVATIONAL STUDY

58 Experimental or intervention study

59 Experimental study Put afford to control the situation of study factors and observe effect of them Control extraneous factors by holding those factors fix or randomization to make two groups have the same distribution of extraneous factors. One group of subject is given experimental treatment and another group (control) is given either none or less preferable. After a period effect of exposure is measured and compared between two groups.

60 Randomization Randomization is used to allocate subject: subject get treatment by chance “double blinding”: One important method to make the result more valid is called “double blinding”: neither investigator nor subjects know the treatment to which they have been allocated Experimental study

61 Limitation: Effect is observed in artificial setting because the controlling extraneous factors Randomization may not be ethical Apart from pharmaceutical intervention, other intervention is difficult to do double blinding Experimental study

62 Summary of the study design

63 Reviewed time

64 ECOLOGICAL STUDY  Exploratory study:  Exploratory study: if there is no specific exposure of interest or the exposure of potential interest is not measure  Analytic study:  Analytic study: if the primary exposure variable is measured and included in the analysis

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66 Main feature of each type of epidemiologic research  Experimental : artificial manipulation of study factor with randomization artificial manipulation of study factor with randomization  Quasi-experimental study : artificial manipulation of the study factor without randomization artificial manipulation of the study factor without randomization  Observational : no artificial manipulation of the study factor no artificial manipulation of the study factor

67 Steps in a prospective cohort study  Define the population at risk (=cohort)  Determine exposure to a factor of interest of all subjects in the cohort  Follow exposed and non-exposed forward in time to ascertain whether they develop the outcome of interest  Compare the outcomes in the exposed and the unexposed group with each other

68  Start with cases.  Determine exposures.  Are the observed exposures higher than expected?  Comparison group (controls) needed to find out what is expected. Steps in case-control study