STUDY DESIGNS

CHP400:Community Health Program- lI

Research Methodology

Observational / Descriptive Studies

Case reportCase series

Cross sectionEcological

Present: Disease Past: Exposure

Cross - section Cohort

Case - control

Present:Disease & Exposure

Present: Exposure Future: Disease

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Time is Key

CONTENT

Classification of Study Designs Observational Studies Descriptive Studies

o Case Reportso Case Serieso Cross-sectional studies (Health Survey)o Ecologic studies

STUDY DESIGNS

ExperimentalObservational

Animal

Experiment

HumanInterventio

nClinical trial

Analytical

Case control

Descriptive

Case report

Case series

Cross section

Ecological

Cohort

Descriptive Studies

Case Reports

Careful and detailed report by one

or more clinicians of the profile of

a single patient (few cases)

•e.g. previously un-described disease

•e.g. unexpected link between diseases

•e.g. unexpected new therapeutic effect

•e.g. adverse events

Case Reports cont.

• A case report gave a clue that

“oral contraceptives use increases

the risk of venous thromboembolism”.

o “Luck” in being the first to encounter an

interesting case.

o Rigor in diagnosis, testing and

documentation of clinical findings

Tendency to publish reports of

strange conditions that have little

relevance to daily practice.

Some authors erroneously try to

imply causation, therapeutic

benefits, etc.

Case Reports

Strengths

over one million case reports indexed on Medline.

uses language that is familiar to clinicians and

easy to interpret.

useful reminder about conditions, diagnoses etc.

that are rarely seen in most practices.

for researchers, case reports generate hypotheses

that can be tested using other study designs.

Case Reports

• No appropriate comparison group.

• Cannot be used to test presence of a statistical

association.

• Since based on the experience of one person:

--- presence of any risk factor

may be purely coincidental

--- Not a true epidemiologic design.

Limitations

Authors: Smart ER. Macleod RI. Lawrence CM.Title: Allergic reactions to rubber gloves in dental patients: report of three cases.Source: British Dental Journal. 172(12):445-7, 1992 Jun 20.

Abstract: Three cases of allergy to rubber are described. The patients exhibited peri-oral rashes following dental treatment by personnel wearing latex rubber gloves. Two of the patients were aware of possible allergy to domestic rubber products but did not reveal this as part of their medical history. With the increase in numbers of dentists wearing rubber gloves it is probable that there will be many more such cases reported in the future. Rubber products must then be added to the list of potential allergens which may be of some importance to the practice of dentistry.

Case Reports

Experience of a group of patients with a

similar diagnosis.

Cases may be identified from a single or

multiple sources.

Generally report on new/unique condition.

May be only realistic design for rare

disorders

Case Series

Descriptive Studies

• Useful for hypothesis generation.

• Used as an early means to identify the beginning or presence of an epidemic.

• Can suggest the emergence of a new disease (i.e. AIDS).

• Informative for very rare disease with few risk factors.

Strengths:

Case Series

Limitations:

• Lack of an appropriate comparison

group

• Cannot be used to test for presence of

statistical association

• Not a true epidemiologic design.

Case Series

Descriptive Studies

Cross-sectional studies

An observational design that surveys exposures and disease status of a population at single point in time

time

Study only exists at this point in time

time

Study only exists at this point in time

Studypopulation

Not Diseased

Diseased

factor present

factor absent

factor present

factor absent

Prevalence

Cross-sectional studies

Often used to study conditions that

are relatively frequent with long

duration (nonfatal, chronic

conditions).

It measures prevalence, not

incidence of disease.

Cross-sectional studies

Strengths:

• Provides prevalence estimates of exposure and disease for a well-defined population.

• Easier to perform than studies that require follow-up (hence relatively inexpensive).

• Can evaluate multiple risk (protective) factors and health outcomes at the

same point in time.

Cross-sectional studies

Strengths:

• May identify groups of persons at high

or low risk of disease

• Can be used to generate hypotheses

about associations between predictive

factors and disease outcomes

Cross-sectional studies

Limitations :

• Prevalent rather than incident (new) cases are used – the exposure could be associated with survival after disease occurrence, rather than development of the disease

• Temporal sequence between exposure and disease cannot be established

i.e. Which came first, chicken or the egg?

Cross-sectional studies

Limitations : cont.

• Not suitable for studying rare or highly

fatal diseases or a disease with short

duration of expression.

Cross-sectional studies

Health Survey

Survey (n): Information gathered by asking a group of individuals the same questions related to their characteristics, attributes.

  Survey (v): The process of collecting such

information

Reach a large number of respondents;

Generate standardized, quantifiable

data - as well as some qualitative data;

and

offers confidentiality / anonymity

A good survey has the potential to :

Surveys can be:

o Descriptive:

These surveys do exactly what they say -

they describe. The goal is to get a snapshot -

of your ‘respondents’

 Explanatory:

These surveys go beyond description and

attempt to establish why things might be the

way they are

o Census: A census surveys every single

person in a defined or target

population .This is a survey that does not

rely on a sample.

 Cross-sectional surveys: This type of survey

uses a sample or cross-section of those

respondents selected to represent a target

population

Surveys can involve population or samples of populations:

Surveys administration :

o Face to face

o Telephone

o Self-administered.

o Electronic e.g. E mail

o comprehensive planning,

o meticulous instrument construction,

o comprehensive piloting,

o Deliberate implementation

o and appropriate analysis

Conducting a survey capable of generating credible data requires:

Survey questions can be : :

o Open questions: Open questions can

generate rich data, but it can be data that

is difficult to code and analyze.

o Closed questions: These questions force

respondents to choose from a range of

predetermined responses, and are

generally easy to code and statistically

analyze.

Considerations in questionnaire Construction:

Providing clear background information

and lucid instructions.

Logical organization.

Comprehensive coverage without undue

length.

Uses of friendly and nice pleasing layout

and design.

• Measures that represent characteristics of

entire populations are used to describe

disease and to postulate causal associations.• Measure of interest is correlation between

exposure rates and disease rates among different groups.

• Correlation coefficient (denoted as “r” )Range is from –1.0 to 1.0Revaluated in relation to difference from

0.

Ecologic studies

Strengths:• Cheap, quick, and simple (generally make use of secondary data)

Limitations:

• Cannot link exposure-disease

relationship at the individual level

• Uses average exposure levels rather

than actual levels of exposure

• Inability to control for confounding

factors

Ecologic studies

Country A Country BPerson Salt Intake Hyp. Salt Intake Hyp. 1 1 Yes 1 Yes 2 1 Yes 1 Yes 3 1 Yes 1 No 4 3 No 1 No 5 3 No 1 No 6 3 No 2 No 7 3 No 2 No 8 5 No 2 No 9 5 No 2 No 10 5 No 2 No

Avg. 3.0 30% 1.5 20%

EXAMPLE: Country A: Country BPrevalence-Hypertension 30% 20%Average Salt Consumption Moderate Low

The “Ecologic Fallacy”:

• wrong conclusions based on grouped data

• Patterns observed on the aggregate level are

not observed on the individual level

Summary

Observational studies are the starting point. Case Reports, Case Series and Cross-sectional

studies are useful for generation of hypotheses. Cross-sectional studies:

-provide prevalence estimates of exposure and disease

-may identify groups of persons at risk of disease