Definitions for each of these study types are taken from:
One of the basic skills required for practicing EBM is developing a well-built clinical question. These questions need to be relevant to patients’ problems and phrased in a way that facilitates your acquisition of relevant and precise answers from the medical literature.
PICO is an acronym or mnemonic that is used to develop a well-built clinical question. It identifies and organizes the key aspects of a complex patient presentation:
PATIENT OR PROBLEM
How would you describe a group of patients similar to yours? What are the most important characteristics of the patient?
INTERVENTION, EXPOSURE, PROGNOSTIC FACTOR
What main intervention are you considering? What do you want to do with this patient?
COMPARISON
What is the main alternative being considered, if any?
OUTCOME
What are you trying to accomplish, measure, improve or affect?
TIME FRAME
What are the time factors involved?
TYPE OF QUESTION
Therapy / Diagnosis / Harm / Prognosis / Prevention / Other
TYPE OF STUDY
Systematic Review / RCT / Cohort Study / Case Control / Other
Background questions concern general knowledge.
These types of questions are usually answered by using a textbook or medical encyclopedia/dictionary.
Foreground questions are specific knowledge questions that affect clinical decisions, including a broad range of biologic, psychological, and sociologic issues.
These are the questions that generally require a search of the primary medical literature and that are best suited to the PICO format.
Center for Evidence-Based Medicine. [cebmed]. (2013, January 28). Finding the Evidence 1 - Using PICO to formulate a search question. [Video file]. Retrieved from https://www.youtube.com/watch?v=ypYkaKnn8_4
An observational study is a study in which the investigator cannot control the intervention or assign treatment to subjects because the participants or conditions are not being directly assigned by the researcher.
In an experimental study, the investigators directly manipulate or assign participants to different interventions or environments.
Case-control studies start by identifying persons with and without a disease of interest (cases and controls, respectively) and then look back in time to find differences in exposure to risk factors. They are a type of observational study that is good to study rare diseases or diseases with a long latency (since you identify those cases at the outset), and can determine multiple exposures.
In cohort studies, groups of individuals, who are initially free of disease, are classified according to exposure or non-exposure to a risk factor and followed over time to determine the incidence of an outcome of interest. In a prospective cohort study, the exposure information for the study subjects is collected at the start of the study and the new cases of disease are identified from that point on. In a retrospective cohort study, the exposure status was measured in the past and disease identification has already begun.
Studies in which the presence or absense of disease or other health-related variables are determined in each member of a population at one particular time.
A quantitative method of combining the results of independent studies, which are drawn from the published literature, and synthesizing summaries and conclusions.
A clinical trial involving one or more new treatments and at least one control treatment with specified outcome measures for evaluating the intervention. The treatment may be a drug, device, or procedure. Controls are either placebo or an active treatment that is currently considered the "gold standard". If patients are randomized via mathematical techniques then the trial is designated as a randomized controlled trial.
A review which endeavors to consider all published and unpublished material on a specific question. Studies that are judged methodologically sound are then combined quantitatively or qualitatively depending on their similarity.
The type of question will often dictate the best study design to address the question:
| Question | Study |
| Clinical Examination | Prospective, blind comparison to Gold Standard |
| Diagnostic Testing | Prospective, blind comparison to Gold Standard |
| Prognosis | Cohort Study > Case Control > Case Series |
| Therapy | RCT is really the only way we want to answer this question. |
| Etiology / Harm | RCT > Cohort Study > Case Control > Case Series |
| Prevention | RCT > Cohort Study > Case Control > Case Series |
| Cost | Economic Analysis |
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