RESEARCH DESIGNS
EXPERIMENTAL DESIGNS AND
RANDOMIZED CONTROLLED TRIALS
INTRODUCTION
In health research the concept of internal validity
is related to the design of research projects and
the extent to which the implementations of these
designs enables researchers to unambiguously
identify causal relationships between
interventions and outcomes.
 Experiments are an important form of
intervention studies. A well-designed experiment
enables researchers to demonstrate how
manipulating one set of variables (the
independent variables) produces systematic
changes in another set of variables (the outcome
or dependent variables).

THE CONCEPT OF CAUSALITY
Three simple criteria for demonstrating
causal associations are:
1. The cause must precede (occur before)
the effect.
2. The cause and effect co-vary. If the
cause occurs then so does the effect.
3. If the cause does not occur, then the
effect does not occur.
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We must attempt to eliminate plausible
alternative explanations or hypotheses that offer
rival causal explanations for the findings.
For example, pain might persist even after the
injury has healed. There might be other variables
operating which maintain a person’s experience
of pain.
The problem is that, apart from our intervention,
there are likely to be other factors that may be
influencing the pain outcome.
 These influences are called ‘confounding
extraneous variables’ or ‘bias’ and constitute a
potentially serious problem when attempting to
interpret the results of a trial.

CONFOUNDING AND BIAS
IN RESEARCH
STUDIES
The research designs that are commonly referred
to as ‘before–after’ designs are often used initially
to test the safety and efficacy of novel
interventions.
 The use of the design provides a clear example of
how confounding variables can threaten the
internal validity (or accuracy) of the study.

EXAMPLE
As a hypothetical example, consider the
introduction of an exercise program for cardiac
patients, which aimed at increasing mobility, and
thereby improving health.
 The patients were also smokers, and were
strongly encouraged to give up smoking.
 The researchers used ‘distance walked by
patients’ (in a specified time period) as an
indicator of the effectiveness of the program.

Previous Figure represents (without showing
numbers) the average walking distances before
and after the exercise program.
 There is clearly a difference between before and
after (i.e. an improvement from baseline).
 However, was this improvement caused only by
the exercise program, or by confounding
extraneous variables, or a combination of both?

CONSIDER THE FOLLOWING PLAUSIBLE ALTERNATIVE
EXPLANATIONS FOR THE DIFFERENCE SHOWN IN
FIGURE :
1.
2.
3.
The improvement might have been due to
natural recovery.
The improvement might have been due to other
factors, such as the reduction in, or cessation of,
smoking among some of the patients, resulting
in an average increase in walking distance.
The improvement might have been due to a
placebo effect.
THE USE OF CONTROL GROUPS
IN APPLIED
HEALTH RESEARCH
A control group consists of participants that
undergo the same conditions as the group
receiving the intervention under investigation.
 For example, in drug trials, control group
participants will often receive an injection of
saline solution.
 If the experimental treatment is administered
via injection, in order to control for the effects of
actual injection.


If the medication were administered orally,
similar looking inert tablets would be used for
control participants. It has been found that, if
people receive any form of ‘therapy’, improvement
may occur even when the ‘treatment’ or
intervention is physiologically and chemically
inert. This is referred to as a placebo effect.
RANDOMIZED CONTROLLED TRIALS
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The protocol being followed has been developed
by the CONSORT Committee, to promote the
standardization of RCTs.
Definition of the population.
Selection of the sample.
Defining the intervention and control groups.
Assignment procedures.
Administration of intervention (treatment).
Measurement of outcomes.
Statistical analysis and interpretation of the
outcomes.