Study Design

• The design of a study is the investigator’s plan of action for answering the research question(s).

• The objective in selecting a study design is to minimize possible errors by maximizing the reliability and validity of the data.

Hurricane Katrina

•? Why were warning ignored

•Why this devastation happened •? What is happening

•? How many victims

•? What could be the consequences

Classification of Study Design

• It can be classified into 3 subheadings

1. Descriptive

1. Analytical

3. Interventional

Cross-sectional

Longitudinal

Case-control

Cohort

Experimental

Quasi-experimental

What type of study will we do?

• It depends on

1.the research question

2. study objective

3. feasibility

If the Research Question is • who is affected/

• What do they know, believe,thinks about the problem

And the research objective is

• To recognize the problem and to set up a hypothesis

We will go for a Descriptive type of study.

Cont.....• R.Q:

1. Are certain factors indeed associated with the problem?

2. What is the cause of the problem?

• so we want to demonstrate the cause and effects

We will go for an Analytical type of study.

Cont......R.Q:

• 1. Will the removal of a particular factor prevent or reduce problem?

• 2.What is the effect of a particular intervention/ strategy?

To test the effectiveness of control measures in small scale project drug trial.

We will go for an Experimental type of study.

• Descriptive study:

1. Merely describe a problem in terms of time,place and person.

2. It doesn’t have a formal comparison group.

3. As there is no comparison group there is no scope for analysis.

4. It can only done for establishing hypothesis.

cont......

• 5. No conclusion can be drawn about the association between exposure and outcome.

Descriptive study has many name,

• Cross-sectional

• Survey

• Prevalence study

Advantages:

• 1. Generalizability.

• 2. It takes short time.

• 3. low cost.

Limitations:

• Cause and effect relationship cannot be measured.

• Some of the diseases of long duration shows a high prevalence rate.

• Chance of missing of acute diseases like asthma.

• Patient who are under treatment can be missed.

• Longitudinal Studies

When observations are repeated on the same population over a period of time.

Exp: serveillance.

• Case- Studies

• Case control study

Population at risk

Disease

sample with disease/cases

sample without disease/controls

Yes

No

Yes

No

Exposure to risk factor

T I M E

Research

selection of cases and controlsInclusion criteria for the mother were as

follows: 1.Woman who gave birth to live- birth singleton

infants.2. Woman having normal vaginal delivery. Exclusion criteria for the mother were as

follows: 1. Non antenatal care card holders.2.Those who gave birth to premature babies.3.Subjects who gave birth to still born babies.

Cont....4. Pregnant women who had any medical

complication (e.g Diabetes Mellitus, Heart Disease , Chronic Lung Disease, Jaundice etc).

5. Eclamptic and pre-eclamptic subjects.6. Multiple pregnancies.7. Caesarian section cases. 8. Congenital abnormal babies.

9. Post mature babies.

Advantages:• Short duration• easier• less expensive• suitable to investigate rare diseases about

which little is known/ also for common disease.

• no risk of subjects• require comparatively few subjects• risk factor can be identified.• Ethical problems are minimal.

Disadvantages:

• problem of bias

• Selection of control group

• we cannot measure incidence rate only odds ratio

• representative ness of cases and control

Odds Ratio

• The odds that a case is exposed divided by the controls is exposed.

Cases Noncases

Exposed A B

Not Exposed

C D

A+B

C+D

A+C B+D

Odds ratio=

[{A/(A+C)}/ {C/ (A+C)}]/

[{B/(B+D)}/ {D/ (B+D)}]

Cohort study

Population at risk

Renal failure

sample

Exposure to NSAIDS

T I M E Population

sample

No

Yes

No

Yes

No

T I ME

Yes

Advantages:

• estimate risk or rate directly because of the availability of population at risk.

• Temporal relationship

• Reduction of Bias

• rare exposure

• Multiple outcome

Disadvantages:

• large sample size

• losses to follow up

• multiple exposure

• ethical problems

• cost

• time

type of cohort study • Prospective• RetrospectiveExp:Records of exposure of members

of the armed services to radio-active fallout at nuclear bomb testing sites are now being used to examine the possible causal role of fall-out in the development of cancer over the past 30 years.

• Retro-prospective

Relative Risk• RR= (Incidence rate among the exposed)/

(Incidence rate among the exposed)

· If RR = 2, There is twice chance of occurrence of the disease among the exposed then the unexposed.

· If RR = 1, There is equal chance of occurrence of the disease among the exposed then the unexposed.

· If RR = 1<, The exposure is rather protective.

Intervention studies

• When researcher manipulates a situation and measures the effect of manipulations.

• types:1. Experimental 2. Quasi-ExperimentalExperimental• only type of study design that can

actually prove causation.• individuals are randomly allocated to

at least two groups.• Manipulation is there.• Randomization.

2. Quasi-Experimental

• all the three main characteristics are not followed.

• Manipulation must be present.

quasi-experimental control group

Study group before

control group before

Intervention Study group after

Control group before

Compare

quasi-experimental no control group

Study group before

Intervention Study group after

Compare

• In drug test,

we measure

sensitivity and specificity of the test.

• sensitivity : The proportion of people with the disease who have a positive test for the disease.

Rarely miss people with the disease.

• specificity:The proportion of people without the disease who have a negative test for the disease.

Rarely misclassify people without the disease as diseased.

Potential Error in Epidemiological Studies

• Errors can be

1. Random Error

2. Systematic Error

Random Error: is the divergence,due to chance alone, of an observation on a sample from the true population value, leading to lack of precision in the measurement of an association.

• There are 3 major sources of Random Error

1.Individual Biological variation.2. Sampling Error.3. Measurement Error.• Sampling error occurs during the

process of selecting study participants who are always a sample of a larger population and

best way to reduce it by increasing the sample size.

• Matching is the process by which we select controls

in such a way that they are similar to cases with regard to certain pertinent selected variables which are known to influence the outcome of disease aetiological factor and which if not matched act as a confounder.

• Systematic error(or bias) occurs in Epidemiology when there is a tendency to produce results that differ in a systematic manner from the true value.

• Systematic error is a particular hazard because epidemiologists usually have no control over participants in studies unlike the situation in laboratory experiments.

• It is difficult to have representative samples of source population.

• Some variables of interest are difficult to measure, like personality type, alcohol consumption habits and past exposures to rapidly changing environmental conditions .

• The principal Systematic Error• 1. selection Bias:occurs when there is a

systematic difference between the characteristics of the people selected for a study and the characteristics of those who are not.

• 2. Measurement bias: occurs when the individual measurements or classifications of disease or exposure are inaccurate.

( they do not measure correctly what they are supposed to measure).

Sources of Measurement bias

• Biochemical or physiological measurements are never accurate and different laboratories often produce different results on the specimen.

• Recall Bias in case-control study.

• Validityis an expression of the degree to

which a test is capable of measuring what it is intended to measure.

•a study is valid if its results correspond to the truth,

• there should be no systematic error and random error should be as small as possible.

• Internal validity- is the degree to which the results of an observation are correct for the particular group of people being studied.

• Internal validity can be threatened by all sources of systematic error but can be improved by good design and attention to detail.

• External validity or generalizability is the extent to which the results of a study apply to people not in it.

• External validity is assisted by study designs that examine clearly stated hypothesis in well defined people.

Ehical Issue

Descriptive

c/s

Descriptive

Longitudinal

Information taken single time

Information taken several time

Descriptive

Longitudinal

cohort

Not cohort Cohort

• 1. persons who are having high fat diet are 2.5 times more prone to develope bowl cancer than those of not having high fat diet.

• 2. High fat diet in takers are 2.5 times more at risk of developing bowl cancer than the non high fat diet in takers.

pop

bowl ca

cases

control

Diet habitContains of the food

+

_

_

+

cases

Cohort

High fat diet

Not high fat diet

+

_

+

_

_

pop

Bowl ca