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Types of Questions and Types of Studies

Types of Questions:

Two additional elements of the well-built clinical question are the type of question and the type of study. This information can be helpful in focusing the question and determining the most appropriate type of evidence or study.

The most common types of questions related to clinical tasks are:

The type of question is important and can help lead you to the best study design:

Types of Studies:

This is one example of what is often referred to as the evidence pyramid or the hierarchy of study design. It is used to illustrate the evolution of the literature. The base of the pyramid is where information usually starts with an idea or laboratory research. As these ideas turn into therapies and diagnostic tools they are tested with laboratory models, then in animals, and finally in humans. The human testing may begin with volunteers and go through several phases of clinical trials before the therapy or diagnostic tool can be authorized for use within the general population. Controlled trials are then done to further test the effectiveness and efficacy of a drug or therapy. As you move up the pyramid the amount of available literature decreases, but increases in its relevance to the clinical setting.  As you move up the pyramid the study design is more rigorous and allows for less bias or systematic error that may distract you from the truth.
           
Case series and Case reports consist of collections of reports on the treatment of individual patients or a report on a single patient. Because they are reports of cases and use no control groups with which to compare outcomes, they have little statistical validity.

Case control studies are studies in which patients who already have a specific condition are compared with people who do not have the condition. The researcher looks back to identify factors or exposures that might be associated with the illness.  They often rely on medical records and patient recall for data collection. These types of studies are often less reliable than randomized controlled trials and cohort studies because showing a statistical relationship does not mean than one factor necessarily caused the other. A case control study starts with patients who already have the outcome and looks backwards to possible exposures.

Cohort studies take a large population who are already taking a particular treatment or have an exposure, follow them forward over time, and then compare them for outcomes with a similar group that has not been affected by the treatment or exposure being studied. Cohort studies are observational and not as reliable as randomized controlled studies, since the two groups may differ in ways other than in the variable under study.  A cohort study starts with the exposure and follows patients forward to an outcome.

Randomized, controlled clinical trials are carefully planned projects that introduce a treatment or exposure to study its effect on real patients. They include methodologies that reduce the potential for bias (randomization and blinding) and that allow for comparison between intervention groups and control groups (no intervention).  A randomized controlled trial is an experiment and can provide sound evidence of cause and effect.  A RCT randomly assigns the exposures and then follows patients forward to an outcome.

Systematic Reviews usually focus on a clinical topic and answer a specific question. An extensive literature search is conducted to identify studies with sound methodology. The studies are reviewed, assessed, and the results summarized according to the predetermined criteria of the review question.

A Meta-analysis will thoroughly examine a number of valid studies on a topic and combine the results using accepted statistical methodology to report the results as if it were one large study.  The Cochrane Collaboration has done a lot of work in the areas of systematic reviews and meta-analysis.

The pyramid serves as a guideline to the hierarchy of study design. You may not always find the highest level of study to answer your question. In the absence of the best evidence, you then need to consider moving down the pyramid.

Other types of Clinical trials:

Clinical Trial [Publication Type]
Work that is the report of a pre-planned clinical study of the safety, efficacy, or optimum dosage schedule of one or more diagnostic, therapeutic, or prophylactic drugs, devices, or techniques in humans selected according to predetermined criteria of eligibility and observed for predefined evidence of favorable and unfavorable effects. While most clinical trials concern humans, this publication type may be used for clinical veterinary articles meeting the requisites for humans. Specific headings for specific types and phases of clinical trials are also available.

Clinical Trial, Phase I [Publication Type]
Work that is the report of a pre-planned, usually controlled, clinical study of the safety and efficacy of diagnostic, therapeutic, or prophylactic drugs, devices, or techniques based on a small number of healthy persons and conducted over the period of about a year in either the United States or a foreign country.

Clinical Trial, Phase II [Publication Type]
Work that is a report of a pre-planned, usually controlled, clinical study of the safety and efficacy of diagnostic, therapeutic, or prophylactic drugs, devices, or techniques based on several hundred volunteers, including a limited number of patients, and conducted over a period of about two years in either the United States or a foreign country.

Clinical Trial, Phase III [Publication Type]
Work that is a report of a pre-planned, usually controlled, clinical study of the safety and efficacy of diagnostic, therapeutic, or prophylactic drugs, devices, or techniques after phase II trials. A large enough group of patients is studied and closely monitored by physicians for adverse response to long-term exposure, over a period of about three years in either the United States or a foreign country.

Clinical Trial, Phase IV [Publication Type]
Work that is a report of a planned post-marketing study of diagnostic, therapeutic, or prophylactic drugs, devices, or techniques that have been approved for general sale after clinical trials, phases I, II, and III. These studies, conducted in the United States or a foreign country, often garner additional data about the safety and efficacy of a product.

Cross-sectional studies describe the relationship between diseases and other factors at one point in time (usually) in a defined population. Cross sectional studies lack any information on timing of exposure and outcome relationships and include only prevalent cases.

Qualitative Research answers a wide variety of questions related to human responses to actual or potential health problems.The purpose of qualitative research is to describe, explore and explain the phenomena being studied. 

Studies that show the efficacy of a diagnostic test are called prospective, blind comparison to a gold standard study. This is a controlled trial that looks at patients with varying degrees of an illness and administers both diagnostic tests -- the test under investigation and the "gold standard" test -- to all of the patients in the study group. The sensitivity and specificity of the new test are compared to that of the gold standard to determine potential usefulness.

Retrospective cohort (or historical cohort) follows the same direction of inquiry as a cohort study.  Subjects begin with the presence or absence of an exposure or risk factor and are followed until the outcome of interest is observed.  However, this study design uses information that has been collected in the past and kept in files or databases.  Patients are identified for exposure or non-exposures and the data is followed forward to an effect or outcome of interest.

Revised July 2010