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To further complicates difficulties with defining “(clinical) trial” , cohort studies can include control group (as discussed in this classic article on distinguishing case series from cohort studies https://annals.org/aim/article-abstract/1033260/distinguishing-case-series-from-cohort-studies) I think the difficulties arise from conflating ethical with methodological/design issues. A (clinical) trial” is indeed an experiment (on humans) and include intent to somehow manipulate/control the environment/intervention PROSPECTIVELY for the purposes of inferring the effects of such a manipulation. An investigator then decides which design is the most suitable to reliably address the goals of the investigation. Thus, RCT, cohort studies, cross-sectional studies all fit this definition of clinical trial. The NIH definition of a clinical trial captures these issues probably the best: “A research study1 in which one or more human participants2 are prospectively assigned3 to one or more interventions4 (which may include placebo or other control) to evaluate the effects of those interventions on health-related biomedical or behavioral outcomes. 3The term “prospectively assigned” refers to a pre-defined process (e.g., randomization) specified in an approved protocol that stipulates the assignment of research subjects (individually or in clusters) to one or more arms (e.g., intervention, placebo, or other control) of a clinical trial. 4 An “intervention” is defined as a manipulation of the subject or subject’s environment for the purpose of modifying one or more health-related biomedical or behavioral processes and/or endpoints. Examples include: drugs/small molecules/compounds; biologics; devices; procedures (e.g., surgical techniques); delivery systems (e.g., telemedicine, face-to-face interviews); strategies to change health-related behavior (e.g., diet, cognitive therapy, exercise, development of new habits); treatment strategies; prevention strategies; and diagnostic strategies.” Ben Sent from my iPad - excuse typos and brevity On Jan 23, 2020, at 4:30 AM, David Nunan <[log in to unmask]> wrote: ________________________________ [Attention: This email came from an external source. Do not open attachments or click on links from unknown senders or unexpected emails.] ________________________________ Thanks Tom. It also gets interesting when you look at how other sectors, borrowing to some degree from medical sciences, use these terms. Here’s the latest: https://www.alliance4usefulevidence.org/the-experimenters-inventory-a-new-resource-for-policy-makers-and-professionals/?preview=true&_thumbnail_id=7962<https://urldefense.com/v3/__https://www.alliance4usefulevidence.org/the-experimenters-inventory-a-new-resource-for-policy-makers-and-professionals/?preview=true&_thumbnail_id=7962__;!!Fou38LsQmgU!6DgBUEjifgQSlMU8oJ_a2hlsLe7CURIdvo6Z-STbkZu8zCBOU8CmRF5ieyyh2OvsIg$> Sent from my iPhone On 23 Jan 2020, at 09:57, K Hopayian <[log in to unmask]> wrote: Agree, Tom. Those are textbook definitions as taught to me at undergrad level. Just one nuance, a cross-sectional study of diagnostic accuracy can be experimental but is not a trial, so an experiment to assess an intervention meant to change course of disease. Kev Hopayian On 23 Jan 2020, at 09:49, Tom Jefferson <[log in to unmask]<mailto:[log in to unmask]>> wrote: To me a trial is an experiment, a clinical trial is an experiment on humans only, then there may be randomisation, blinding etc but these should be described as features further refining the design. All trials are controlled (conducted) by investigators to an extent, with randomisation being one extreme. Case series are what comes through the surgery door either in all cases or particular types of cases. 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