Systematic Review: A summary of evidence, typically conducted by an expert or expert panel on a particular topic, that uses a rigorous process (to minimize bias) for identifying, appraising, and synthesizing studies to answer a specific clinical question and draw conclusions about the data gathered. (Level I)
Meta-analysis: A process of using quantitative methods to summarize the results from the multiple studies, obtained and critically reviewed using a rigorous process (to minimize bias) for identifying, appraising, and synthesizing studies to answer a specific question and draw conclusions about the data gathered. The purpose of this process is to gain a summary statistic (i.e., a measure of a single effect) that represents the effect of the intervention across multiple studies. (Level I)
Clinical Practice Guidelines: Systematically developed statements to assist clinicians and patients in making decisions about care; ideally, the guidelines consist of a systematic review of the literature, in conjunction with consensus of a group of expert decision makers, including administrators, policy makers, clinicians, and consumers who consider the evidence and make recommendations. (Level I)
Randomized Controlled Trials (RCTs): A true experiment (i.e. one that delivers an intervention or treatment in which subjects are randomly assigned to control and experimental groups); the strongest design to support cause-and-effect relationships. (Level II)
Quasi-experiments: A type of experimental design that tests the effects of an intervention or treatment but lacks one or more characteristics of a true experiment (e.g., random assignment; a control or comparison group). (Level III)
Cohort Studies: A longitudinal study that begins with the gathering of two groups of patients (the cohorts), one that received the exposure (e.g., to a disease) and one that does not, and then following these groups over time (prospective) to measure the development of different outcomes (diseases). (Level IV)
Case-control Studies: A type of research that retrospectively compares characteristics of an individual who has a certain condition (e.g. hypertension) with one who does not (i.e., a matched control or similar person without hypertension); often conducted for the purpose of identifying variables that might predict the condition (e.g., stressful lifestyle, sodium intake). (Level IV)
Cross-sectional Studies: A study designed to observe an outcome or variable at a single point in time, usually for the purpose of inferring trends over time. (These do not have a designated level as these are "a moment in time" and used in many different study designs).
Meta-synthesis: A rigorous process of analyzing findings across qualitative studies. The results address a specific research question and are obtained through the synthesis of qualitative studies. The process allows researchers to find greater meaning through interpreting the qualitative data. (Level V)
Descriptive Studies: Those studies that are conducted for the purpose of describing the characteristics of certain phenomena or selected variables. (Level VI)
Qualitative Studies: Research that involves the collection of data in non-numeric form, such as personal interviews, usually with the intention of describing a phenomenon. (Level VI)
Case Study/Case Reports/Case Series: An intensive investigation of a case involving a person or small group of persons, an issue, or an event. Reports describe the history of a single patient, or a small group of patients, usually in the form of a story. (Level VII)
Experiential and non-research evidence: Literature review, quality improvement, program or financial evaluation. (Level VII)
Background Information/Expert Opinion/Evidence Summaries: Fully referenced expert topic reviews written by recognized authorities who review the topic, synthesize the evidence, summarize key findings, and provide specific recommendations. (Level VIII)