====== Level of Evidence 1 ======

A high degree of clinical [[certainty]]

Level A

Based on consistent Class I evidence (well-designed, prospective
randomized controlled studies)

Level B

Single Class I study or consistent
Class II evidence or strong Class II
evidence especially when circumstances preclude randomized
clinical trials

[[Randomized controlled trial]] (RCT) of the [[intervention]] of interest with masked or objective outcome assessment, in a representative [[population]]. Relevant [[baseline]] characteristics are presented and substantially equivalent among treatment groups or there is an appropriate statistical adjustment for differences.
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Clinicians' trust [[level of evidence 1]] [[recommendation]]s, issued on preponderantly solid [[randomized clinical trial]]s (RCTs), to guide best practice [[decision-making]]. However, sometimes physicians following one [[clinical practice guidelines]] (CPG) find themselves in a situation in which they do not follow another, issued on the same strong evidence base. 
The aim of Volovici et al. is to reflect on the [[consistency]] of [[recommendation]]s in different [[guideline]]s (between-guideline consistency). They also consider within-guideline consistency (or durability), defined as the number of recommendations carried over from one edition to another in consecutive editions of the same CPG. For illustration purposes, they use two examples: hypertension guidelines and traumatic brain injury (TBI) guidelines. They conclude that just like research, CPGs also need to have between-guideline and within-guideline consistency (akin to the [[reproducibility]] of studies). Clinicians and researchers should take into account the lower [[consistency]] of guidelines that are not based on at least one strong RCT
((Volovici V, Steyerberg EW. Lost in translation between [[evidence]] and [[recommendation]]s: Expert opinion is needed to define "level I". World Neurosurg. 2021 Mar 25:S1878-8750(21)00465-4. doi: 10.1016/j.wneu.2021.03.095. Epub ahead of print. PMID: 33775869.)).
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[[Level of Evidence]] 1

[[Evidence]] obtained from at least one properly designed [[randomized controlled trial]].

Level 1a

Evidence from large randomized clinical trials (RCTs) or systematic reviews (including meta-analyses) of multiple randomized trials which collectively has at least as much data as one single well-defined trial.

Level 1b

Evidence from at least one “All or None” high quality cohort study; in which ALL patients died/failed with conventional therapy and some survived/succeeded with the new therapy (for example, chemotherapy for tuberculosis, meningitis, or defibrillation for ventricular fibrillation); or in which many died/failed with conventional therapy and NONE died/failed with the new therapy (for example, penicillin for pneumococcal infections). 

[[Level of Evidence 1C]].


Level 1d

Evidence from at least one RCT.
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The [[guidelines]] by the American Heart Association Stroke Council for treatment of [[chronic hydrocephalus]] secondary to [[subarachnoid hemorrhage]] by the permanent [[Cerebrospinal fluid shunt]] is presented as a [[Class I]] [[recommendation]] but is based upon [[Level of evidence C]] 
((Connolly ES, Jr., Rabinstein AA, Carhuapoma JR, et al. Guidelines for the
management of aneurysmal subarachnoid hemorrhage. Stroke. 2012;43(6):1711-
1737.))