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Thanks, Rod A point well taken, which clarifies the issue what is trial and what is not. I actually intended to refer to cohort studies (which, as you well know it, is often considered as one of the paradigmatic designs for observational research). However, it seems that we should then distinguish cohort, non-randomized trials (when investigator has say in determining exposure) vs cohort-observational studies (when the researcher does not have say in allocating exposure). I have not seen this distinction being made before, but it looks like that Tom’s post forced us to clarify this epidemiological conundrum (again, I base my reasoning on this classic paper, I mentioned before : https://annals.org/aim/article-abstract/1033260/distinguishing-case-series-from-cohort-studies) Best Ben Sent from my iPad - excuse typos and brevity On Jan 25, 2020, at 9:04 PM, Rod Jackson <[log in to unmask]> wrote: Hi Ben. By definition, if the investigator has a say in treatment, then it’s not an observational study, rather it’s a trial. Sounds like you are describing a non randomised trial. Cheers Rod * * * * * * * * sent from my phone On 26/01/2020, at 15:25, Benjamin Djulbegovic MD <[log in to unmask]> wrote: Rod, I have actually been involved in the studies where “the investigators in an observational study HAD say what drugs the participants take.” In fact , this is a rather common occurrence in (cancer) clinical research ... How would you otherwise classify the study in which exposure is determined by the investigators but the study is not randomized ? But, I agree that the intent and control seem to define what is a clinical trial- design is irrelevant. ( if the investigators have say, then it is a trial , if they don’t then it is not) Best Ben Sent from my iPhone (Please excuse typos & brevity) On Jan 25, 2020, at 17:51, Rod Jackson <[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.] ---------------------------------------------------------------------- A study is called an observational study when exposure status (e.g. on drug A or drug B or no drug) is observed (measured) by the investigator whereas in a randomised trial the investigator organises for participants to be randomised to drug A or drug B or perhaps a placebo. In the observational study example, a clinician unassociated with the study has usually prescribed drug A or drug B to participants as part of usual care and all the study investigator does is ‘observe’ / measure what drugs the participants are taking. The investigator in an observational study has no say what drugs the participants take. The word ‘controlled’ in randomised controlled trials simply refers to the fact that participants are randomly allocated to the study intervention or to a comparison or ‘control’ group. A control group is actually common in both observational studies and RCTs, but in RCTs participants are allocated to groups at random. Cheers Rod Jackson On 26/01/2020, at 12:15 PM, Moacyr Roberto Cuce Nobre <[log in to unmask]> wrote: I do not understand Observational Study as a no controlled study. Some people are exposed and some are not. _______________________________________ Moacyr Roberto Cuce Nobre, MD, MS, PhD Equipe de Epidemiologia Clínica e Apoio à Pesquisa Instituto do Coração (InCor) Hospital das Clínicas Faculdade de Medicina da Universidade de São Paulo 55 11 2661 5941 (fone/fax) 55 11 991 331 009 (celular) Em qui., 23 de jan. de 2020 às 07:51, Tom Jefferson <[log in to unmask]> escreveu: Dear EBMers, thank you to all those who responded to my to questions. I have collated the answers verbatim whenever I could. They are summarised in a table which I will send to interested requestors. I have a few comments and would be grateful if you could add to them. What strikes me is the heterogeneous nature of the answers. I used 2 vague terms "trial" and "clinical trial" and purposely did not mention terms like "controlled", "placebo", "randomised" and few respondents did. Despite their vagueness, the terms are in everyday use and I believe their definitions should be tigthened. Alarm bells started going off in my head when I saw case series and even retrospective cohort studies referred to as "trials". One of the biggest culprits in this mess is, alas, clinicaltrials.gov. People who are planning to conduct and report case series register them and then seemingly feel entitled to call them "trials" as the register bears that name. The whole terminology needs tightening up to avoid misrepresentation and the usual reporting bias creeping in. 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. Observational studies have no control (conduction) exerted. I hope you find this discussion interesting. Tom. 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