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Descriptive and Ecological studies
Reza Majdzadeh,
Professor of Epidemiology
School of Public Health
Tehran University of Medical Sciences
Learning objectives:
Uses of descriptive studiesSteps of descriptive studies
Rational for Ecologic studiesCommon errors in Ecological studies
Descriptive studies:
PersonTimePlace
MICSANISIMESDHSWorld Health SurveyNon communicable diseases risk factors study (Stepwise)National Health Survey
Descriptive studies:
PersonTime
Secular trend, Cyclic changesEpidemic curves
Place
TREND OF TB INCIDENCE RATE ( I.R.IRAN , 1962-2001 )
0
20
40
60
80
100
120
140
160
YEAR
INC
IDE
NC
E
RA
TE
( /
10
00
00
)
QUARTER /
4.54.03.53.02.52.01.51.0.5
stan
dard
ized
fre
qu
en
cy o
f TB
in
sm
ear
posi
tives
120
110
100
90
80
70
TimeTime
Descriptive studies:
PersonTime
Secular trend, Cyclic changesEpidemic curves
Common sourcePropagatedExtended source
Place
Distribution of Cholera Cases in the Golden Square Area of London, August – September 1854
John Snow (1850s)
Mapping Techniques
Dot maps
Using geographic areas as the unit of analysis
Proportional density maps
Isopleth maps – portray areas of equal measures, can show diffusion (weather maps)
Population Distribution
(a) Geographical distribution and (b) with size of District re-drawn to be proportional to population size
Rose et al. Eurosurveillance 2003; 8:1
Influenza diffusion pathways, using harmonic measures with isopleths.
Real World
Person:
Place:
Time:
Who
?
Where?
When?
PPT Sheet
Beliefs concerning dietary pratices during pregnancy and lactation: A qualitative study among Iranian women residing in Sweden.Impact of the maternal psychological disorders on the self-esteem of the teenagers in a context of immigration (comparative study of the iranian families in Sweden and in Iran)Where do I belong?: The experience of second generation Iranian immigrants and refugeesImmigrant women challenge the role of men: How the changing power relationship within Iranian families in Sweden intensifies family conflicts after immigrationEthnicity, self reported psychiatric illness, and intake of psychotropic drugs in five ethnic groups in SwedenImpact of ethnicity, violence and acculturation on displaced migrants: Psychological distress and psychosomatic complaints among refugees in SwedenHTLV-I from Iranian Mashhadi Jews in Israel is phylogenetically related to that of Japan, India, and South America rather than to that of Africa and MelanesiaPsychiatric problems among Iranian immigrants in CanadaEthnicity, acculturation, and self reported health. A population based study among immigrants from Poland, Turkey, and Iran in Sweden.
• Sampling• Design Effect• Sample size
Simple Random Sampling Stratified Sampling Cluster Sampling Multi-stage Sampling
• ICC: Intra-Cluster Correlation
• Design effect:
1+ (m-1) ICC
Variance of cluster design/
Variance of simple random
Ecologic Studies
Performance of Golestan province health centers
شبكه هاي بهداشتي درماني در استان گلستان:تسهيالت مركزبد مركزبدشهري واحد
زايماني روستايي شبانهروزي روزانه پايگاهبهداشتي خانهبهداشت شهرستان
- 3- 2- 19بندرگز
- 5- 2- 18كردكوي
1 8 1 3- 33تركمن
1 8 1 2- 45آققال
- 12- 8 11 75گرگان
- 6- 4 1 42عليآباد
4 22 3 12 2 152گنبد
1 8 1 3- 66مينودشت
2 10 2 1- 68كالله
9 82 8 37 14 518جمعكل
Ecologic
Analysis—Plots/Correlation coefficients
Cross-checking incidence estimates: Americas + Eastern Mediterranean
1
10
100
1000
30 35 40 45 50
Average age ss+
Est
imat
ed s
s+ i
nci
den
ce
Iran
Lebanon
Bahrain
Jordan
Rational
1. Low cost and convenience2. Measurement limitations of
individual level studies3. Design limitations of individual-
level studies4. Interest in ecologic effect5. Simplicity of analysis and
presentation
Ecologic
WeaknessEcologic FallacyConfounders
Example : High positive correlation between suicide rates and largely Protestant populations. Suicide actually committed by the Catholics in the community who were socially isolated.
250
200
150
100
50
0
0 1600140012001000800600
Japan
Denmark
New Zealand
Fed. Repub.Of Germany
France
Canada
Israel
Switzerland
USA
Australia
Yugoslavia
Hong KongRomania
Finland
Poland
Spain
Hungary
Norway
UKItaly
Sweden
Incid
en
ce R
ati
o p
er
10
0,0
00
Wom
en
Per Capita Supply of Fat Calories
Correlation between dietary fat intake and breast cancer by country.
Prentice RL, Kakar F, Hursting S, et al: Aspects of the rationale for the Women’s Health Trial. J Natl Cancer Inst 80:802-814, 1988.)
Is there a relationship between breast cancer incidence and dietary fat consumption by country?From the graph , we see that as average dietary fat
consumption increases, breast cancer incidence increases.
What is wrong with this data?
The problem is: the ecologic fallacy!
Prentice et al. J Natl Cancer Institute 1988 80:802-814
The ecologic fallacy
Attributing to members of a group characteristics that they do not possess as individuals
In our example, we only know average values of fat consumption by country - we don’t know if individuals with breast cancer had higher fat intake
Ecologic study
Comparison of groups NOT individuals
1. Measurement2. Analysis3. Inference
EcologicMeasures
Aggregate-summarizes characteristic within a group (proportion of homes with T.V.s)Environmental-describes physical characteristic of a geographic location (hours of sunlight/darkness)Global- can’t be measured on an individual level (seatbelt law)
Aggregated; observations derived from individualsEnvironmental; data has analogue in individual level with heterogeneityGlobal; no distinct analogue in individual level
Inference
Biologic; wearing helmet and mortality of motorcycle riders
Ecologic; helmet-use laws on mortality of riders in different states
The target level of casual inference does not always match the level of
analysis.Cross-level inference
Ecologic studies
A study in which the units of analysis are populations or groups of people, rather than individuals.
Usually takes advantage of pre-existing data collected for other purposes- an efficient and economical study design
No time element- a “snapshot” of populations- think cross-sectional studies of populations, not individuals
CPR
10090807060504030
CBR
40
30
20
10
0
Contraceptive Prevalence Rate
Cru
de B
irth
Rat
e تقسيم بندي واحدها از نظر كارآيي يا
EfficiencyZone 1
Zone 2
Zone 3
Zone 4
Low efficiency
High efficiency
Medium efficiency
Medium efficiency
Learning objectives:
Uses of descriptive studiesSteps of descriptive studies
Rational for Ecologic studiesCommon errors in Ecological studies
Information is data touched by the human mind !
A. Einstein