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