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Study Design

Prof Md Anisur Rahman (Anjum)Prof & Head of the department Ophthalmology. Dhaka Medical

College. Dhaka

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What is research design?

• “A research design is the arrangement of condition for

collection and analysis of data in a manner that aims

to combine relevance to the research purpose with

economy in procedure”.

It is a conceptual structure

Blueprint for the collection, measurement and analysis of

data.

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study design What is Clinical study design?

Clinical study design is the formulation of trials and

experiments in medical and epidemiological research

Types of study designs:

There are no hard and fast classification of study design

In broad scene we classify study design into two groups

1) Observational study and 2) Experimental study.

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Purposes of study design The purposes we will consider include:

1) Describing the prevalence of health problems;

2) Identifying causes of health problems (etiological research),

and

3) Evaluating therapy, including treatment and prevention

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Two types Study Design

Study DesignExperimental (The

researcher intervenes to change

reality, and then observes what

happens)

Observational (The

researcher studies, but not alters

what occurs)

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Observational study:Observational study: 4 things to be discuss:

1) When do we do an observational study?

2) Some key points of observational study

3) Types of observational study

4) Goal of observational study

Another name of observational study is Non interventional study or descriptive study.

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When do we do an observational study?

a) When we merely want to collect descriptive information: "Is

the incidence of diabetes rising?“

b) When we want to report on the causes of a problem without

disturbing the natural setting (I want to find out why students

do not attend lectures)

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When do we do an observational study?

c) When we can't do an experiment: "How fast does the earth

move around the sun?“

d) When it's not acceptable to do an experiment: "How much

does not wearing a condom increase the likelihood of HIV

infection

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Some key points of observational study:

1) Simply describe the problem e.g. pattern of disease occurrence, distribution of disease, general view of disease.

2) Provide general information about the relations of

occurrence and distribution of disease in population with

basic characteristics e.g. age, gender, religion, occupation,

education, areas, time, marital status, habit, social class etc.

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Some key points of observational study:

3) No research question and no attempt for answering the

research question.

4) Usually no hypothesis and don’t seek explanation or cause.

5) It is based on observation of naturally occurring events

created by natural experiments.

6) A sample of population is observed for various characteristics

by interview, questionnaire, measurement, records etc.

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Some key points of observational study:

7) How exposure occurs in natural setting. Nature determines who is exposed to the exposure and who not. Researcher does not determine the assignment of exposure; only passively observe the events that unfold.

8) Subjects are not randomized to the exposed or unexposed groups, rather the subjects are observed in order to determine both their exposure and their outcome status and the exposure status is thus not determined by the researcher.

9) It is regarded as precursor of analytic study.10)Not structured formally like analytical and experimental

study.

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Types of observational study

Observational study are of four main types;

1) Case-series,

2) Case-control,

3) Cross-sectional (including surveys), and

4) Cohort studies.

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Case-series studyDefination: When certain characteristics of a group (or series) of

patients (or cases) are described in a published report, the result

is called a case-series study.

It is the simplest design in which the author describes some

interesting or intriguing observations that occurred for a small

number of patients.

Case-series studies frequently lead to the generation of

hypothesis.

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Cohort studies are used to study incidence, causes, and

prognosis. Because they measure events in chronological order

they can be used to distinguish between cause and effect.

Case controlled studies compare groups retrospectively. They

seek to identify possible predictors of outcome and are useful

for studying rare diseases or outcomes. They are often used to

generate hypotheses that can then be studied via prospective

cohort or other studies.

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Cross sectional studies are used to determine prevalence.

They are relatively quick and easy but do not permit

distinction between cause and effect.

Objective Common design

Prevalence Cross sectional

Incidence Cohort

Cause (in order ofreliability)

Cohort, case-control, cross sectional

Prognosis Cohort

Treatment effect Controlled trial

16Sunday, February 09, 2014 anjumk38dmc@gmail.com

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Case-control, cross-sectional & cohort study

These three types of studies (case-control, cross-sectional &

cohort study) are defined by the period of time the study

covers and by the direction or focus of the research question.

• Cohort and case-control studies generally involve an extended

period of time and it needs to take data several times

• For this reason, both cohort and case-control studies are

sometimes also called longitudinal studies.

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Case-control, cross-sectional &cohort study

• The major difference between them is the direction of the

inquiry or the focus of the of the research question.

• Cohort studies are forward-looking, from a risk factor to an

outcome, whereas case-control studies are backward-looking;

from an outcome to risk factors.

• The cross sectional study analyzes data collected on a group of

subjects at one time.

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Goal of observational study:

1) Alert community about the disease

2) Provide clue to etiology & hypothesis

3) Helps in planning of health care facilities

4) Focus on who, what, when & how questions.

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Did investigator assign exposure?

NOEXPERIMENTAL STUDY

YESOBSERVATIONAL STUDY

Random Allocation? Comparison Group?

YES NO NOYES

Randomized Control Trial

NON Randomized Control Trial

ANALYTIC STUDY

DESCRIPTIVE STUDY

Exposure to OutcomeCohort study

Outcome ExposureCase-control study

Exposure and outcome at the same time Cross-sectional study

According to time direction

Study design tree

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Difference between descriptive study and analytic

studyDescriptive study Analytic studyGenerate hypotheses Test hypotheses

Answer what, who, where, and when

Answer why and how

Diffuse & superficial with research question & answering but provide useful insight into the problem

Narrow down to a specific research question & its answering by more rigorous study design & data analysis

No attempt to analyze the link betn exposure & outcome

Analyze the link betn exposure & outcome

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Difference between descriptive study and analytic

study (Contd……)Descriptive study Analytic study

Alert community about the disease

Prove etiology & determinants of disease

No comparison group Needs comparison group

Less ethical constrains

Thorny ethical issue

More chance of bias. Less chance of bias.