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Tuesday, March 11, 2008

Major purpose of randomization in a clinical trails

050. The major purpose of randomization in a clinical trails is to:

1. Facilitate double blinding

2. Help ensure the study subjects are representative of general population

3. Ensure the groups are comparable on base line characteristics

4. Reduce selection bias in allocation to treatment


3. Ensure the groups are comparable on base line characteristics


Park 18th Edition Page 76

Wolff N. Randomised trials of socially complex interventions: promise or peril? Journal of Health Services Research & Policy 6(2):123-126, 2001.

Pawson R and Tilley N. Realistic evaluation. Sage Publications, London, 1997.

Black N, Brazier J, Fitzpatrick R and Reeves B (eds).








Analytic study designs can be divided into two groups; experimental and observational.


o Randomised controlled trial


o Cohort

o Case-control

o Cross-sectional

In experimental studies, the assignment of individuals to an intervention or treatment is under the control of the investigator.

The randomised controlled trial (RCT)

The RCT is considered the “gold standard” of study designs. It consists of two major components:

(a) A control group, which makes sure that any outcome effects in the intervention group not due to the intervention can be measured and adjusted for;

(b) Random assignment of participants to the treatment and control groups, which makes sure that bias is minimised, and potentially confounding variables are distributed evenly across the two groups.

(c) A third component often considered is blinding, which makes sure that neither the participant nor the researcher knows which group the participant has been assigned to.

A trial with good internal validity is one in which any effect seen can be attributed to the intervention. A trial with good external validity is one in which the results can be readily generalised to the target population. RCTs, by their design, are ensured good internal validity, but are often lacking in external validity due to the strictly defined

inclusion and exclusion criteria.

The intervention

In socially complex interventions, the intervention is often not explicitly defined and is often changed during the trial. Unlike clinical trials of new drugs, clients are offered a range of interventions which they might or might not be able to or want to use, depending on, for example, their entrenched habits, level of risk, and capacity for learning.


Randomisation is the heart of the trial. It will give the greatest confidence that the groups are comparable so tht like can be compared with like. Ideally, each subject should be assigned at random to intervention or control group. In socially complex interventions, especially those involving General Practice, it is neither feasible nor practical to have one GP look after both control and intervention clients. In fact there are often circumstances in which randomisation is unnecessary, unsuitable, misleading or impossible. In these circumstances non-randomised observational studies can have a vital role in providing evidence for the effectiveness of the intervention, as long as careful statistical analysis is undertaken.

Benefits of Randomised Controlled Trials

Ä Strong level of evidence

Ä Can relate effects to causes

Ä Good internal validity

Ä Randomisation, control group and blinding reduces the risk of bias and confounding

Drawbacks of Randomised Controlled Trials

Ä Randomisation to treatment/exposure may not always be possible or ethical, for example, in a study of smoking and lung cancer, an investigator could not ethically ask a group of subjects to smoke

Ä Socially complex interventions are not ideally suited to the RCT design, as several of the criteria needed to ensure validity, reliability and generalisability are not found in such trials.

Ä May have limited external validity

Hierarchy of evidence

Generally speaking, RCTs are considered to provide stronger evidence than cohort studies, which themselves provide stronger evidence than case-control studies. However, the hierarchy of evidence is merely a convenient rule of thumb that, all other things being equal, RCTs are more able to attribute effects to causes. In fact high quality RCTs and high quality observational studies often produce similar results.


Randomisation is an attempt to eliminate “bias” and allow for comparability


Ä In observational studies, the investigator follows or observes participants, but cannot assign them to an intervention. Observational studies provide weaker quantitative evidence than do experimental studies because of the potential for confounding to be present.


Ä Cohort studies (also called prospective or longitudinal studies)

o In a cohort study a group of people are divided into those exposed or not exposed to a particular risk factor or intervention, and followed up over time. The proportion of the exposed group who develop a disease or outcome is then compared with the proportion in the unexposed group. Cohort studies are susceptible to bias by differential loss to follow-up.

Ä Case-control studies

o In this type of study, a group of people with a particular disease (cases) is matched with a group of people without the disease (controls). Their background history is then examined to determine the proportion of cases who, in the past, had been exposed to a risk factor, compared with the proportion of controls who had been exposed. These studies are particularly useful for rare conditions or for risk factors with long induction periods. Case-control studies are prone to recall bias, and often the choice of control group is problematical.

Ä Cross-sectional studies

o These are descriptive studies (surveys), which examine the relationship between diseases and other risk factors at one point in time in a defined population. Cross-sectional studies lack any information on timing of exposure and outcome relationships.

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