descriptive and cross sectional studies

8/10/2019 Descriptive and Cross Sectional Studies

1/40

Epidemiological study designs

1


2/40

The life cycle of epidemiological investigations:

The never ending story

Identifying

data needs

Spelling out the

research question

Formulating the

study objectives

Planning the

analysis

Preparing data

collection instruments

Analysing data

Drawing conclusions

Formulating

recommendations

Involving the

programme

Collecting data

Here we

go again


3/40

Descriptive Studies


4/40

Epidemiologic Study Designs

Grimes & Schulz, 2002 (www)4Descriptive studies
http://dx.doi.org/10.1016/S0140-6736(02)07283-5http://dx.doi.org/10.1016/S0140-6736(02)07283-5


5/40

Epidemiologic Study Designs

Grimes & Schulz, 2002 (www)5Descriptive studies

Descriptive

study
http://dx.doi.org/10.1016/S0140-6736(02)07283-5http://dx.doi.org/10.1016/S0140-6736(02)07283-5


6/40

Hierarchy of Epidemiologic Study Designs

Tower & Spector, 2007 (www)
http://taylorandfrancis.metapress.com/content/n708745v34251883/http://taylorandfrancis.metapress.com/content/n708745v34251883/


7/40

Descriptive studies 7

Start ofHIV/AIDSResearch

New outbreak of

oral tumors,

malignancies and

infectious diseases

strikes young male

homosexuals

CDAJ, 1982Squamous cell

carcinoma in sexualpartner of Kaposi

sarcoma patient

Lancet,1982

AIDS in the "gay"

areas of SanFrancisco

Lancet, 1983


8/40

Meticulous observations by

Burkitt

Epstein Barr Virus

BurkittsLymphomaDescriptive studies 8


9/40

Descriptive study - Definition

A study concerned with and

designed only to describetheexisting distribution ofvariables without regard to

causal or other hypotheses(John M. Last).

9Descriptive studies


10/40

escriptive Studies

Are often the first step inepidemiological investigation

Allow natureto take its course

The simplestof epidemiologicalstudies

Often stimulate the initiationfor moredetailed studies



11/40

Descriptive Study VsAnalytical Study

Descriptive Study Analytical Study

Concerned with characteristicsthe amount and distributionof disease within a population

Concerned with determinants ofdisease, reasons for relatively

high or low frequency ofdisease in specific population

subgroups

Used to identify any healthproblem that may exist

Proceed to identify the causesof the problem

Generateshypothesis Testshypothesis

Precedesanalytical study Followsdescriptive study



12/40

Prevalence of smoking in adultmen in South India

Country

Percentage of smokers

Urban Rural

Tamilnadu 66 88

Kerala 88 84

Karnataka 76 41

Andhrapradesh 57 75



13/40

Prevalence of Flourosis, India



14/40

Procedures in Descriptive studies

1. Defining the populationto bestudied

2. Defining the diseaseunder study

3. Describing the diseaseby person,place & time

4. Measurementof disease

5. Comparingwith known indices6. Formulation of an etiological

hypothesis



15/40

1 Defining the population to be studied

Investigations of populations notindividuals

Total number, age, sex, occupation, etc.,

Whole population or a representativesample or a selected group

The chosen community should be stable

A close health facility

Defined population is the Denominator



16/40

2. Defining the disease under study More investigators / interviewers may be involved

Epidemiologist Operational definition

Precise Clearly indicates who is a case

Valid The identified case should actually have the disease

Acceptable to the population Too many decline consent Applicable to large numbers

Eg. Malaria

Epidemiologist frames his own definition if necessary

Adherence


3 D ibi h di b


17/40

3. Describing the disease byperson, place & time

Person Place Time

Age, Sex, Height,Weight

Country, Region Year, Season

Birth order, Familysize

Urban, Rural, Localcommunity

Month, Week

Occupation,

Education, socialstatus

Towns, Cities Day, Hour of onset

Blood pressure,Blood cholesterol

Institutions Duration



18/40

4. Measurement of disease

Mandatory to know the Burden ofillness.

Morbidity, mortality, disability etc.,

Mortality straightforward

Morbidity two aspects

Incidence from longitudinal studies

Prevalencefrom cross sectionalstudies



19/40

5. Comparing with known indices

Essence of epidemiology

Between different populations,

subgroups of same population

Provides clues to etiology

Also identifies groups which are atincreased risk for certain diseases


l f


20/40

6. Formulation of anetiological hypothesis

Hypothesis- supposition arrived at fromobservation or reflection that can be tested bycollecting facts that lead to their acceptance

or rejection. It is a refutable prediction. Hypothesis should be complete and specific

a. Population

b. Specific cause considered

c. Expected outcome

d. Dose response relationship

e. Time response relationship



21/40

For Example:

Cigarette smoking causes lung

cancer



22/40

Smokingof30 40cigarettes per daycauses

lung cancer in 10% ofsmokers after20 years of

exposure



23/40



exposure


POPULATION


24/40



exposure


SPECIFIC

CAUSE

POPULATION


25/40



exposure


OUTCOME

SPECIFIC

CAUSE

POPULATION


26/40



exposure


OUTCOME

DOSESPECIFIC

CAUSE

POPULATION


27/40



exposure


OUTCOME

DOSESPECIFIC

CAUSE

POPULATION

TIME-RESPONSE

RELATIONSHIP

f


28/40

Types of Descriptive Studies

Case Report & Case Series

Descriptive Cross-sectional

Studies (Community Surveys)

Ecological Descriptive Studies



29/40

Case Report & Case Series

A case study (case report) is a methodof descriptive research thatdocuments a practitioners

experiences, thoughts, orobservations related to the care of a

singleclient of a specific condition.

A case series combines theobservations from a group of similarclients



30/40

Benefits of case studies

Convey detailed information that mightbe lost in a clinical trial throughdocumentation of actual events andinterventions in clinical practice

Are faster and less expensive tocomplete than clinical trials

Provide a rationale for furtherinvestigation



31/40

Descriptive Cross-sectional Studies(Community Surveys)

Simplest of descriptive studies

Based on a single examination of across-section of population at one point

of time. Also known as Prevalence survey.

A survey with questions used to describea population

Do you take vitamins?

What vitamins do you take?

When do you take them?32Descriptive studies


32/40

Ecological Descriptive Studies

When the unit of observation is anaggregate (e.g. family or school) of anecological unit (a village, town or

country) the study becomes anecological descriptive study.

Eg:Esophageal cancer rates in Japan

Eg: Occurrence of higher number ofrespiratory illness in different citieswith different levels of pollution.



33/40

Uses of Descriptive studies

Provide data regarding the magnitude ofthe disease load and types of diseaseproblem in the community in terms ofmorbidity and mortality rates

Provide clues to etiology Provide background data for planning,

organizing and evaluating preventive andcurative services

Contribute to research by describingvariations in disease occurrence byPerson, Place & Time



34/40

Cross-Sectional Studies

Begin with Cross-sectional sample

Determine Exposureand Diseaseat same

time


35/40

Cross-sectional study


36/40

Cross-sectional studies


37/40

Cross-sectional No direction

Exposure

(Risk Factor)

Disease(Outcome)+

+

_

_


38/40

Ill Non-ill Total

Exposed a b a+b

Non-exposed c d c+d

Total a+c b+d a+b+c+d

Presentation of the data of a cross-sectional

study in a 2 x 2 table

Known simultaneously when the study is completed


39/40

Risk measurement from a

cross-sectional study

Prevalence of illness among exposed and

compare it with that of non-exposed

a/a+b /c/c+d Prevalence of exposure among ill and

compare it with that of non-ill

a/a+c /b/b+d


40/40

Thank you!

D i i di 41

descriptive and cross sectional studies

Documents