“An expert is one who knows more and more about less and less until he knows absolutely everything about nothing.”

Study Designs

There are different types of clinical research. The design used in any study is dependent on the

question being investigated. This section will look at some of the most commonly used study
Cohort studies
Case control studies
Cross sectional surveys
Case reports
Clinical trials
Cohort Studies
This type of study design follows specific groups of people (e.g. those with a disease,
with exposure to a treatment or who are alike but for one lifestyle factor such as
occupation or high alcohol consumption etc.) over a period of time to see how
outcomes differ when they receive different treatments and interventions.
This study design allows you to collect specific data over a given time, and allows for interaction
with participants. However cohort studies can be expensive and can take years to complete.
Examples of research carried out using this type of design can be seen in the papers:
Observational cohort study of HIV-infected African children by Laufer et al (2006).
Do we adequately respect the potential of routine primary health care services in reducing
neonatal mortality in developing countries? The example of the Denizli cohort by Uner et al
Case Control Studies
In these studies cases are matched to measure the impact of an intervention or disease. People
with a specific disease or experiencing a specific intervention (the cases) are compared with
individuals who do not have the disease or have not experienced the intervention (the controls).
Once matched data is collected from both groups, it is then compared to investigate whether other
characteristics (e.g. nutrition) are different.
The aim is to try to uncover what caused the outcome in one group but not the other. To reduce
the number of factors that could be related to the difference in cases and controls, all participants
should be recruited from the same population.
This study design can be subject to recall bias (events can be reported differently between the two
groups e.g. those with the disease may remember interventions or exposures more easily than
those in the control group). However this design is relatively inexpensive, can be completed in a
short period of time and can be carried out by individuals or small groups of researchers.
Examples of research carried out using this type of design can be seen in the following papers:
Results from an international case-control study of childhood brain tumours: the role of
prenatal vitamin supplementation by Preston-Martin et al (1998).
Changing patterns in demography of cleft lip-cleft palate deformities in a developing country:
the Smile Train effect – what lies ahead? by Patil et al (2011).
Cross Sectional Surveys
These surveys are used to gather information from a specified number of cases (e.g. individuals,
systems, etc) in a population at a single point in time. The design is used to examine factors
expected to remain unchanged during the period of data collection. These surveys are a basic
variety of descriptive or observational epidemiology.
Mosby’s Dictionary states that:
“this study design aims to describe the relationship between diseases (or other health-related
states) and other factors of interest as they exist in a specified population at a particular time,
without regard for what may have preceded or precipitated the health status found at the time of
the study.”
This study design is relatively quick and easy but it does not allow for a distinction to be made
between cause and effect.
Examples of research carried out using this type of design can be seen in the papers:
Quality of pharmacies in Pakistan: a cross-sectional survey by Butt et al (2005).
Prevalence of stroke and related burden among older people living in Latin America, India and
China by Ferri et al (2011).
Case Reports
Case reports are the detailed histories of single cases. They provide the
symptoms, signs, diagnosis, treatment, and follow-up of an individual patient.
Vandenbroucke (2010) listed six topics that most case reports focus on and
these are:
unexpected associations between diseases or symptoms
unexpected events in the course of observing or treating a patient
findings that shed new light on the possible pathogenesis of a disease or an adverse effect
unique or rare features of a disease
unique therapeutic approaches
a positional or quantitative variation of the anatomical structures
The advantage of this type of design is that case reports allow us to uncover unexpected effects,
new diseases, etc., which can lead to increased knowledge. However, as case reports are
essentially ‘anecdotal’ they are the lowest level of scientific evidence, because they have not been
gained through rigorous scientific study.
Examples of research carried out using this type of design can be seen in the papers:
Occupational asthma in Korea by Oh & Kim (2010).
Obstructive jaundice at the initial presentation in small-cell lung cancer by Ochi et al (2010).


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