overview of study design

8/4/2019 Overview of Study Design

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Overview of Study Design

Epidemiological studies can be broadly classified as either: observational studies or interventional studies

Epidemiological studies can also be categorised according to whether they compare: groups or individuals

Observational studies can be further classified depending on whether they are: descriptive or analytical

In this session we will briefly introduce you to each of these different study designs.You will learn about the main design features of each type of study. You will also learn whenit is appropriate to carry out each type of study. This information will help you to

understand the following sessions.

At this stage we do NOT expect you to understand the details of how to carry out andinterpret each type of study. You will learn how to do this later.

Cross Sectional Studies

In a cross-sectional study, we measure the frequency of a particular exposure(s) and/oroutcome(s) in a defined population at a particular point in time.

Cross-sectional studies can either be descriptive or analytical.

In a descriptive cross-sectional study, we simply describe the frequency of the exposure(s)and/ or outcome(s) in a defined population.

Example 1We could use a descriptive cross-sectional study to describe the prevalence of smoking in aparticular community at a particular point in time.

Example 2

We could use a descriptive cross-sectional study to describe the prevalence ofantibodies

to hepatitis B virus in a particular community at a particular point in time.

In an analytic cross-sectional study, we simultaneously collect information on both the outcome of interest and exposure to potential risk factor(s).followed by the analysis

We then compare the frequency of the outcome in the people exposed to each risk factor

with the frequency in those not exposed.

Example 1

In an analytic cross-sectional study on smoking we could simultaneously collect informationon the smoking habits of each individual in a community and a number of personal

characteristics, such as age, sex, and occupational status.


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We could then compare the prevalence of smoking between men and women, and in

different age and occupational groups.

Example 2

In an analytic cross-sectional study on hepatitis B in a particular community, we couldsimultaneously collect data on the presence of antibodies to hepatitis B infection and on

potential risk factors for infection, such as blood transfusion, intravenous drug use andsexual behaviour.

Cross-sectional studies are especially useful for defining the health needs of a population at

a particular point in time, and as the first step in more detailed etiological investigations.

However, as they are based on prevalent (existing) cases rather than incident (new) cases,they are of limited value for investigating aetiological relationships.

ExerciseSuppose you were carrying out a cross-sectional study looking for an association betweenantibodies to dengue virus and the presence ofAedes aegypti

larvae in the home in a small rural community.

How would you collect the data for this study?

Cohort StudiesIn a cohort study we start by selecting a study population or cohort of people who do notinitially have the outcome of interest. We then classify the members of the cohort according

to whether they have been exposed to the potential risk factor or not.

We then follow the entire cohort over time and compare the incidence of the outcome(s) inthe exposed individuals with the incidence in those not exposed.

Example 1In a cohort study looking at the effects of cigarette smoking on incidence of lung cancer, we

would start by selecting a cohort of people without lung cancer. We would then classify themembers of the cohort as smokers or non-smokers.

We would follow the cohort over time and compare the incidence rate of lung cancer in the

smokers with the incidence rate in the non-smokers.

Example 2

In a cohort study of the association between chronic hepatitis B infection and primary livercancer, we would start by selecting a cohort of people without primary liver cancer. Wewould then classify the members of the cohort according to whether or not they have

chronic hepatitis B infection.

We would follow the cohort over time, and compare the incidence risk of primary liver

cancer in people with chronic hepatitis B infection with the incidence risk in people withoutchronic hepatitis B infection.

Cohort studies are particularly useful for rare exposures and in situations where we are

interested in studying more than one outcome.

However, cohort studies are generally slow and expensive to carry out. They are also

inefficient for investigating rare outcomes.


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Rarity of disease and long latency are not contra-indications for cohort studies - their

disadvantages need to be balanced against the superior quality of evidence cohort studies

deliver compared with other study designs, such as case-control studies.

Exercise:Suppose you wanted to carry out a cohort study to investigate the association between lack

of physical activity and the development of osteoporosis in women. How would you selectthe individuals to include in your study?

Case Control Studies

In a case-control study we start by identifying individual cases of the outcome of interest.

We then identify a representative group of individuals who do not have the outcome. Theseindividuals act as controls.

We then compare cases and controls to assess whether there were any differences in theirpast exposure to one or more possible risk factors.

Example 1

In a case-control study looking for an association between cigarette smoking and lung

cancer, we would start by identifying the individual cases of lung cancer within our studypopulation. Then we would select a representative group of people without lung cancer as

our controls.

We would then collect information on the past smoking habits of both the cases and

controls and analyse the data to see if more cases than controls have a past history ofsmoking.

Example 2

In a case-control study looking for an association between chronic hepatitis B infection andprimary liver cancer, we would start by identifying the cases of primary liver cancer withinour study population. Then we would select a representative group of people without

primary liver cancer as our controls.

We would then compare the prevalence ofchronic hepatitis B infection in the cases andthe controls.

The major difference between case-control studies and cohort studies is the way that the

study subjects are chosen. In a cohort study, subjects are selected before they have had a

chance to develop the outcome of interest. They are classified according to their exposureor non-exposure to the possible risk factor. We then follow them over time to see if theydevelop the outcome.

In contrast, in a case-control study we select subjects on the basis of the presence orabsence of the outcome. We then look to see if they were exposed or not to the possible

risk factor.

The big advantage of case-control studies is that past exposures and outcomes aremeasured at the same time. Because of this, these studies can be done quickly, without the

need to wait for the outcome to occur.

Another advantage of case-control studies is that they are a good way to study rare

outcomes and outcomes with a long induction period. This is because the starting point isthe identification of people with the outcome in question


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However, there are several limitations of the case-control study design. We will look at

these limitations in some more detail later.

Exercise:

Suppose you wanted to carry out a case-control study to investigate the associationbetween not breast feeding a baby and infant death due to diarrhoea. How would you select

the individuals to include in your study?

Intervention StudiesIn an intervention study, we do not just observe the distribution of the exposure and

outcome in a population. We actively allocate the exposure (or intervention) to one of the

study groups. The group that is allocated not to receive the intervention acts as a controlgroup. If the intervention is a type of treatment, the control group can be described as

receivingstandard treatment.

We then follow the groups over a period of time. We compare the frequency of the outcomein the group allocated to receive the intervention with the frequency of the outcome in the

group who were allocated not to receive the intervention.

Example 1Suppose we wanted to investigate the effect of nicotine patches in helping smokers to give

up smoking. We would allocate a number of smokers to a treatment group and an equalnumber of individuals to a control group. The treatment group would receive nicotinepatches and the control group would receiveplacebo patches.

We would follow the two groups up over a period of time and compare the frequency ofsmoking in the nicotine patch group and the placebo group.

An inactive intervention, administered to the control group in a controlled trial of anexperimental treatment. The experimental treatment must produce better results than theplacebo in order to be considered effective.

Example 2In an investigation of the effect of vaccination against hepatitis B infection on the incidencerisk of primary liver cancer, a large number of infants were allocated to receive a course of

hepatitis B vaccine and a similar number were allocated not to receive it.

The infants were followed up over a number of decades to determine the incidence risk of

primary liver cancer in the intervention and control groups.

In a community intervention study (or community field trial), whole groups or communities

are allocated to receive the intervention or not.

In most intervention studies, we randomly allocate the individuals or groups to receive the

intervention. This ensures that the allocation of the intervention is determined by chancealone.

Intervention studies provide strong evidence about causation, because when designed well

(with random allocation) they minimise the possibility of selection bias and confounding.

However, for ethical reasons we can only use them to study interventions that are ofpotential benefit to patients, such as preventive or therapeutic measures.


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Exercise

Would it be ethical to carry out an intervention study to test the hypothesis that smoking

causes lung cancer?

Ecological studiesEcological studies compare the exposure status and outcome status of groups rather than

individuals. An ecological study thus looks for an association between an exposure and anoutcome at the group level. In other words, we look to see whether the outcome is more

frequent in groups where the exposure is more frequent.

For each group, the exposures and outcomes are measured for the group as a whole. So, it

is not possible to link the exposure of any particular individual to his or her outcome.

Example 1

In an ecological study of the association between smoking and lung cancer in several cities,we would compare the proportion of the adult population who smoke in each city with theincidence rate of lung cancer for each city.

In such a study, we would not know whether any particular individual who developed lung

cancer was a smoker or not.

Example 2In an ecological study of the association between chronic hepatitis B infection and primaryliver cancer in several countries, we would compare the chronic hepatitis B prevalence rate

in each country with the incidence rate of primary liver cancer for each country.

Ecological studies are the only studies that enable us to investigate the differences betweengroups and the effects of group properties. They can also often be carried out relatively

quickly and cheaply, using routine data.

Because of this, we often use them as a first step in the investigation of a possible

exposure-outcome relationship.

However, it can be difficult to interpret the results of an ecological study. We will look at thisissue in some detail later.

Exercise

Suppose you wanted to carry out an ecological study to investigate the association between

unemployment and coronary heart disease in several regions. How would you carry out thestudy?

Case series

Case series are descriptive studies which focus on individuals. They examine theexperience of a group of individuals sharing a common experience. This may be a shared

exposure (or risk factor) or a shared outcome (perhaps symptoms, or a disease). A keycharacteristic of case series is that there is no comparison group.

Example 1

A study reported on a group of 14 health care workers and hospital support staff infected

with SARS. Individual patient data was used to describe the clinical source, treatment andcomplications they experienced. The 14 individuals were followed for up to three weeks

after discharge from hospital.


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Example 2

Twelve children and adolescents with autism received a new treatment for a minimum

period of six weeks. A validated scale was used to assess any improvement, and negativeside-effects were noted.

Case series are common in clinical medicine, where they are often used to highlight unusual

or interesting findings. They may be used to describe a new disease or a rare manifestationof an existing disease. Case studies can also be used to help detect the effects (adverse or

beneficial) of a new treatment.

Case series can be particularly useful for generating hypotheses, which can then be

explored in larger, well-designed studies.

Case series are attractive as they can usually be conducted quickly and cheaply

Despite their popularity in clinical medicine, case series are less commonly used inepidemiology. This is largely because they are considered to be weak in terms of empirical

evidence. The lack of a comparability can lead to misleading conclusions. In addition, the

number of individuals involved in case series is often small.

Summary

Cross-sectional studiesIn a cross-sectional study, we measure the frequency of a particular exposure(s) and/or

outcome(s) in a defined population at a particular point in time.

In a descriptive cross-sectional study, we simply describe the frequency of the exposure(s)or outcome(s).

In an analytic cross-sectional study, we simultaneously collect information on both theoutcome of interest and on potential risk factors. We then compare the frequency of the

outcome in the people exposed to each risk factor with the frequency in those not exposed.

Cohort studiesIn a cohort study we start by selecting a study population or cohort of people who do not

initially have the outcome of interest.

We then classify the members of the cohort according to whether they have been exposed

to the potential risk factor or not.

We then follow the entire cohort over time and compare the incidence of the outcome(s) in

the exposed individuals with the incidence in those not exposed.

Case-control study

In a case-control study, we start by identifying individual cases of the outcome of interest.We then identify a representative groups of controls without the outcome.

We then compare cases and controls to assess whether there were any differences in their

past exposure to one or more possible risk factors.


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Intervention studies

In an intervention study, we actively allocate the exposure (or intervention) to one of the

study groups. The group that is allocated not to receive the intervention acts as a controlgroup.

We then follow the groups over a period of time. We compare the frequency of the outcome

in the group allocated to receive the intervention with the frequency of the outcome in thegroup who were allocated not to receive the intervention.

Ecological studies

Ecological studies compare the exposure and outcome status of groups rather thanindividuals.

For each group, the exposures and outcomes are measured for the group as a whole. So, itis not possible to link the exposure of any particular individual to his or her outcome.

Case series

Case series describe the experiences of a group of individuals who share similar

characteristics.

Case series have no comparison group, so are purely descriptive. Any findings from caseseries should be interpreted with caution.

REFERENCE

Overview of Study Design (FE08). London School of Hygiene and Tropical Medicine.

September 2010. v 2.0.

overview of study design

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