Types of randomised control trails -Spm from park textbook

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Types of Randomized Controlled Trials (RCTs) - Park's Textbook of Preventive & Social Medicine

Park classifies RCTs under Experimental Epidemiology. The section "Types of Randomized Controlled Trials" lists 5 types:

1. Clinical Trials

These are the most common type of RCT. They are primarily concerned with evaluating therapeutic agents, mainly drugs. They test new treatments, procedures, or services before their wide-scale introduction.

Examples from Park:

Beta-blockers in reducing cardiovascular mortality post-myocardial infarction
Folate supplementation before conception to prevent recurrence of neural tube defects
Aspirin on cardiovascular mortality and beta-carotene on cancer incidence
Tonsillectomy for recurrent throat infection
Coronary bypass surgery for prevention of myocardial infarction

Note: Not all clinical trials are susceptible to blinding (e.g., surgical trials like tonsillectomy, where it is obvious who received surgery).

2. Preventive Trials

These trials aim to prevent or eliminate disease on an experimental basis. They involve primary preventive measures. The most common types are:

Vaccine trials
Chemoprophylactic drug trials

Classic example from Park:

The 1946 MRC (UK) whooping cough vaccine trial: 3,801 vaccinated children vs. 3,757 unvaccinated. Attack rate was 1.45 per 1000 child-months in the vaccinated group vs. 6.72 in the control group.

Since preventive trials involve larger numbers of subjects and often a longer time span, they face greater logistical challenges.

3. Risk Factor Trials

A subtype of preventive trial. The investigator intervenes to interrupt the usual sequence in the development of disease in individuals who already have a known "risk factor." This often involves risk factor modification.

Example from Park:

The Multiple Risk Factor Intervention Trial (MRFIT), also called "Mr. Fit" - a large-scale trial in the USA testing whether reduction of risk factors (hypertension, smoking, hypercholesterolaemia) could reduce coronary heart disease mortality.
The Oslo Diet/Heart Study
The Lipid Research Clinics Study

4. Cessation Experiments

These experiments test whether the removal of a suspected causal factor reduces the risk of disease. They provide strong evidence for causality.

Example from Park:

A trial where one group of cigarette smokers continues to smoke and the other group quits. A decrease in lung cancer incidence in the quitting group greatly strengthens the hypothesis of a causal link between smoking and lung cancer.

5. Trials of Aetiological Agents

These trials involve deliberately exposing human volunteers to a suspected aetiological (causal) agent to test the hypothesis that the agent causes disease.

These are subject to serious ethical constraints and are rarely done. Example: Early studies on transmission of infectious diseases to volunteers.

Summary Table

Type	Purpose	Key Feature
Clinical Trials	Evaluate therapeutic agents/drugs	Most common RCT type
Preventive Trials	Test vaccines/chemoprophylaxis	Larger sample sizes, longer duration
Risk Factor Trials	Interrupt disease development by modifying risk factors	Subtype of preventive trial
Cessation Experiments	Test effect of removing a causal factor	Strengthens causal hypothesis
Trials of Aetiological Agents	Expose volunteers to suspected cause	Rare, ethical constraints

Source: Park's Textbook of Preventive and Social Medicine, Chapter on Experimental Epidemiology - "Types of Randomized Controlled Trials" (block 2, lines 598-690)

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