Question

In about 12-15 sentences, please explain why it is not always possible for epidemiologists to use an experimental design such as the RCT to test an etiologic hypothesis on humans.  

Be sure to fully support your answer with relevant information from the lecture notes, and utilize additional, credible outside sources where needed.

Answer 1

ANSWERS

Randomized controlled trials are inappropriate for the types of questions typically addressed in health promotion research.

It is agreed that for certain questions that arise in the health promotion field, research methodologies other than RCT are indeed more appropriate.

• Randomized controlled trials is a study design that randomly assigns participants into an experimental group or a control group. As the study is conducted, the only expected difference between the control and experimental groups in a randomized controlled trial (RCT) is the outcome variable being studied..
• Epidemiological studies are used to determine the frequency of specific health problems, identify patterns in occurrences of the problem, identify any potential causes and risk factors, evaluate the efficacy of preventative measures and treatments.

It has expanded to include the study of factors associated with non-transmissible diseases like cancer, and of poisonings caused by environmental agents. Epidemiological studies can never prove causation; that is, it cannot prove that a specific risk factor actually causes the disease being studied.

• Experimental studies are less susceptible to confounding because the investigator determines who is exposed and who is unexposed.
• In particular, if exposure is allocated randomly and the number of groups or individuals randomised is large then even unrecognised confounding effects become statistically unlikely.
• There are, of course, ethical constraints on experimental research in humans, and it is not acceptable to expose subjects deliberately to potentially serious hazards. This limits the application of experimental methods in the investigation of disease aetiology, although it may be possible to evaluate preventive strategies experimentally.

For example,

Factories participating in a coronary heart disease prevention project were assigned to two groups, one receiving a programme of screening for coronary risk factors and health education, and the other being left alone. Subsequent disease incidence was then compared between the two groups. The main application of experimental studies, however, is in evaluating therapeutic interventions by randomised controlled trials.

Randomised controlled trials

• At the outset of a randomised controlled trial the criteria for entry to the study sample must be specified (for example, in terms of age, sex, diagnosis, etc).
• As in other epidemiological investigations, the subjects studied should be representative of the target population in whom it is hoped to apply the results.

Epidemiology is the study of diseases in populations of humans or other animals, specifically how, when and where they occur. Epidemiologists attempt to determine what factors are associated with diseases (risk factors), and what factors may protect people or animals against disease (protective factors).

• Epidemiological studies measure the risk of illness or death in an exposed population compared to that risk in an identical, unexposed population (for example, a population the same age, sex, race and social status as the exposed population).

RCTs provide exact and prescriptive protocols to ensure scientific rigor in the most transparent of ways, by randomly allocating treatment. When factors that may bias the estimate of the effect of the intervention on the primary outcome are randomly distributed across intervention and comparison arms of an RCT, there is assurance that results derived are not subject to confounding bias. The allocation of the intervention by the investigators also reduces the probability of selection bias, by assigning people to specified study conditions rather than allowing them to choose.

• Despite the apparent advantages of RCTs, only a small fraction of all human studies use randomized designs. However, much is known about relations between risk factors and disease. Recommendations on the role of human behaviors as determinants of health are made across a wide variety of risk factors and disease outcomes with little or no RCT-derived evidence.

For examples - US Surgeon General's 1964 report on Smoking and Health. On the basis of Hill's Criteria for Judging Causality, the expert panel concluded that RCTs were not necessary to assert that tobacco “causes” an array of health outcomes, including lung cancer. This showed that strong, persuasive evidence can come from sources other than RCTs, which may be difficult or impossible to conduct for a variety of ethical or logistical reasons.

Hence, in many circumstances it is not ethically possible to ascertain the evidences of etiologic hypothesis on humans. Thus the applicability of Randomized Control Trials are minimal with epidemiologic studies.

…………………………………..