The definition of EBM states the importance of obtaining the "best available evidence." The evidence hierarchy attempts to rank the health information resource/study designs, with the highest quality sitting atop and the lower levels descending below. This hierarchy helps guide evidence acquisition (literature searching) and provides more precise communication when discussing the landscape of health literature.
When prioritizing the evidence in any specific situation, clinicians will need to understand the nuances of the evidence (reviews, studies) obtained. It is not uncommon for lower levels of evidence on the hierarchy to trump evidence sitting above a hot-off-the-press large RCT over an older meta-analysis containing small studies (UM-Madison Libraries Research Guides on Evidence-Based Medicine).
Filtered Resources-Searching for evidence-based information begins by looking for the highest level of evidence possible. This process is systematic reviews or meta-analyses where the literature on a topic has already been researched to provide the best answer to a clinical question or practice issue.
Unfiltered Resources-Information that has not been critically appraised is considered "unfiltered." You'll also need to search unfiltered resources (the primary literature) to locate studies that answer your question. Unfiltered resources are individual articles that provide the most recent information from clinical and practice research, such as case studies, comparative studies, or clinical trials. With unfiltered resources, it is up to the nurse to evaluate each study to determine its validity and applicability to the patient or the practice question.