Descriptive Epidem iology & Descriptive Epidem iology & - PowerPoint PPT Presentation

Descriptive Epidem iology & Descriptive Epidem iology & Study design Study design Potjam an Siriarayapon Siriarayapon Potjam an Bureau of Epidem iology Bureau of Epidem iology

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

KEY CONCEPTS ฀ “ “Descriptive epidem iology Descriptive epidem iology” ” 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 . 3

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 . 4

Secular ( long- - term ) trends: term ) trends: to Secular ( long predict or evaluate control program Reported cases (per 1,000) 6 0 0 Vaccine Licensed 5 0 0 4 0 0 3 0 0 2 0 0 1 0 0 0 0 5 0 5 0 5 0 6 6 7 7 8 8 9 9 9 9 9 9 9 9 1 1 1 1 1 1 1 Year 5 Measles by year of report, United States, 1960-1989

Seasonality: to see seasonal pattern to see seasonal pattern Seasonality: over years over years Cases ฀ 1 0 8 9 7 6 9 8 4 9 9 2 0 A p r Onset by Month O c t F e b J u n D e c A u g Cases of eosinophilic meningitis , Nan province , Thailand, 1997-1999 6

Day of w eek and tim e of day: especially especially Day of w eek and tim e of day: important for condition that related to important for condition that related to occupational or environmental condition occupational or environmental condition Deaths 3 0 2 5 2 0 1 5 1 0 5 0 8 9 1 0 1 1 1 2 1 3 1 4 1 5 1 6 1 7 1 8 Time of Day Fatalities associated with farm tractor injuries 7 by time of day , Georgia, 1971-1981

Epidem ic period: to show the time course to show the time course Epidem ic period: of disease outbreak of disease outbreak ฀ Flood Cases 40 30 20 10 0 19 21 23 25 29 1 3 5 7 9 11 13 15 17 19 21 Num ber of leptospirosis patients Date of onset 8 by date of onset , Nov 2 5 -Dec 2 1 , 2 0 0 0

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. 9

Meningococcal cases, Meningococcal cases, Tak province, province, Tak 2 0 0 2 - - 2 0 0 3 2 0 0 3 2 0 0 2 Myanmar Umphang district village Refugee camp Nov.-Dec. Jan-Feb. Mar.

11 Thai-Myanmar border area in Umphang district

12 I n the refugee cam p

13 House of Karen people outside the cam p

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 14

Sex specific attack rates of reported Sex specific attack rates of reported , Songkla , Nov cases , Songkla , leptospirosis cases Nov 25 25- - Dec Dec leptospirosis 15, 2000 15, 2000 C a ses /100,0 00 8 0 5 9 .5 6 0 4 0 2 0 2 0 0 M ale F em a le S ex 15

Age specific specific attack rates of attack rates of reported reported Age , Songkla , Nov cases , Songkla , leptospirosis cases Nov 25 25- - Dec Dec 15, 15, leptospirosis 2000 2000 Cases /100,000 80 58.3 51.1 60 40.7 37.3 33.9 40 21.6 16.7 20 0 0-9 10-19 20-29 30-39 40-49 50-59 60+ Age group 16

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DEFINITION OF EPIDEMIOLOGY 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, 1 9 8 8 ) 18

19 Study design Study design

” ? ? Why “ “ type of study type of study” Why • 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 20

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

22 STUDY DESIGN OF DESCRIPTIVE STUDY

STUDY DESIGN OF DESCRIPTIVE STUDY STUDY DESIGN OF DESCRIPTIVE STUDY � Individual level � Case reports or case series � Cross - sectional surveys of individual � Population level � Ecological study ( Correlational study ) 23

CASE REPORTS AND CASE SERIES CASE REPORTS AND CASE SERIES : describe experience of a Case reports : � Case reports � single patients . Case reports document unusual medical occurrence and can represent the first clues in the identification of new diseases . : collections of individual case Case series : � Case series � reports . Investigation of the activities of the individual in case reports can lead to formulation of a hypothesis . 24

25 Streptococcus suis

CASE REPORTS AND CASE SERIES � Advantage Advantage � � discover new diseases � bring background information to form hypothesis about risk factors � Disadvantage Disadvantage � � might base on only one case � do not have a comparison group 26

- SECTIONAL CROSS - SECTIONAL STUDY STUDY CROSS Total population Cases 27

Character of cross-sectional study � Collect data for all population; case & healthy people � Information obtain is the number of total cases among total population at that study period: prevalence � Can be either descriptive or analytic, depend on design 28

CROSS - SECTIONAL STUDY Descriptive Analytic Descriptive Analytic � Collected num ber of � Exposure and cases and num ber disease status are of total population assessed sim ultaneously � Can assess only prevalence of � Can determ ine disease or other association health events , also betw een exposure and disease called prevalence study 29

Vaccine coverage among Thai children � Proportion of children received vaccine � Education level of mother and vaccine status in children 30

Cross- - sectional study sectional study Cross Defined Population Gather Data on Exposure and Disease Not Not Exposed Exposed Exposed Exposed exposed : Exposed : Do not Do not Have disease have disease Have disease have disease 31

Hypothetical illustration of the interrelationship between an occupational exposure and prevalence of disease 80 well 80 well 100 100 Job A Job A Workers Workers 10 change jobs (hazardous hazardous) ) ( due to illness 20 ill 10 ill 95 well 95 well Job B 100 (non-hazardous) Workers 5 ill 10 ill 10 ill 15 ill 15 ill Point X Prevalence of job A = 20 % ( 20/100 ) Prevalence of job B = 5 % ( 5/100 ) 32 Prevalence ratio Prevalence ratio = 4 = 4

Hypothetical illustration of the interrelationship between an occupational exposure and prevalence of disease 8 0 w ell 8 0 w ell 1 0 0 1 0 0 Job A Job A W orkers 10 change jobs W orkers ( hazardous hazardous) ) ( due to illness 2 0 ill 1 0 ill 9 5 w ell 9 5 w ell Job B 1 0 0 ( non-hazardous) W orkers 5 ill 1 0 ill ill 1 5 ill ill 1 0 1 5 Point Y Prevalence of job A = 1 1 % ( 1 0 / 9 0 ) Prevalence of job B = 1 4 % ( 1 5 / 1 1 0 ) 33 Prevalence ratio = 0 .8 0 .8 Prevalence ratio =

� Advantage � Easy and less time consumed � Estimate magnitude of problem especially for rare disease � First step for exploring and forming hypothesis � Able to use data for further case-control or cohort during analytic study � Disadvantage � Difficult to confirm that risk factor come before disease ( temporal relationship ) � Length biased sampling: Disease that has long duration will over-represent the magnitude of illness while short duration will under- represent illness.

STUDY DESIGN OF DESCRIPTIVE STUDY STUDY DESIGN OF DESCRIPTIVE STUDY � Individual level � Case reports or case series � Cross - sectional surveys of individual � Population level � Ecological study ( Correlational study ) 35

ECOLOGICAL STUDY 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. 36