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Quick guide: common types of medical studies

    Clinical Investigations

EXPERIMENT 1 A researcher assembles two groups of study participants with Lyme Disease. She administers the antibiotic doxycycline to one group and amoxicillin to the other. The researcher then measures which has more of a beneficial effect.
EXPERIMENT 2 A researcher identifies two groups of elderly immobilized patients in a nursing home. One group of patients is respositioned every two hours. The other group is repositioned every 1.5 hours. The researcher measures whether the incidence of bed sores is lower in the group repositioned every 1.5 hours than the group repositioned every 2 hours.
EXPERIMENT 3 A researcher develops a counseling program designed to increase medication compliance in AIDS patients. The researcher delivers the program to one group of patients but not another, then studies whether the group that underwent the counseling program adhered to their medication better than the group that did not.

What do all three experiments have in common?
In each case the researcher actively interfered with one group of study subjects and then compared the outcome with a similar group of subject that did not receive the same intervention. Experiments in which researchers interfere with their study subjects are known as clinical investigations. All examples above pertain to humans, but many clinical investigations are done using animal subjects, particularly for new or risky interventions.

Randomized clinical trials (aka controlled clinical trials) are a common type of clinical investigation.
Randomized clinical trials begin with the assignment of subjects to two groups. As the name randomized implies, participants are randomly assigned to groups. Random assignment insures that subjects in both groups will be equally as likely to develop the disease/condition being tested. In an ideal study, the participants as well as investigators are not aware of which participants are assigned to which group. This is called double-blind assignment. If the patients are “masked” but the investigator knows who is in which group, the study is only single-blinded. After subjects are assigned to two groups, one group is administered an intervention and the other is not. The group that receives the intervention is called the study group. The group receiving no intervention exists for purpose of comparison and is called the control group.

    Observational Studies

EXPERIMENT 1 A researcher identifies a group of patients who were incorrectly diagnosed with Lyme Disease, then records whether the patients experienced adverse effects from the antibiotics they were prescribed for treatment of Lyme.
EXPERIMENT 2 A researcher identifies a group of elderly immobilized patients in a nursing home. The researcher follows the patients for 6 months and records the administration of any topical (skin) agent. At the end of the study period, the researcher conducts an analysis to determine whether certain topical regimens related to reduced incidence of bed sores.
EXPERIMENT 3 A researcher identifies a group of individuals with AIDS who have low medication compliance. The researcher administers a questionnaire to the patients to determine what factors relate to their poor compliance.

What do all three experiments have in common?
In each case the researcher does not actively manipulate or intervene with patients; rather, the researcher merely observes the effect of one or more factors on an outcome. It is easy to see why observational studies received their moniker.

Common types of observational studies:

Case series (aka descriptive study or time series)
The major difference between a case series and a randomized clinical trial is that in a case series all subjects receive the treatment and there is no formal control group. If the researcher uses a comparison group, it is a historical control or (less formally) “prior experience.”

Cohort study
The major difference between a cohort study and a randomized controlled trial is that in a cohort study the subjects determine whether they are to receive the intervention or not. The researcher is not involved with the subject’s decision to use the intervention. When the investigator is identifying participants for the study, they are unaware of which subjects have or will develop the outcome. If the two study groups are observed simultaneously; it is called a concurrent cohort study. Concurrent cohort studies are also known as prospective cohort studies because participants are followed forward in time. If, on the other hand, one group of subjects currently under observations is compared against a historical group, it is called a retrospective or historical cohort study. Historical cohort studies can be engineered by obtaining data (possibly patient records) from previous years.

Case-control study (aka retrospective study)
Case-control studies observe two groups of subjects--one which has developed a disease (case) and another which has not (control). They look back in time to identify factors that might have caused the condition. Neither the case nor the control groups are aware of exposure or non-exposure to the factors being investigated. If the researcher believes the risk factor exists at the same time the condition does, the study is considered to be a cross-sectional case-control study.

    Determining Study Type

Many factors determine what type of study a researcher chooses to use. These often include: reliability of results, study participant availability, bias, cost, ethics, risk factors, and time. For more information on this topic, see the New England Journal of Medicine's site, "How to Determine Your Article Type"

    Try to identify study types in these examples.

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