The basics: what’s the difference between a trial and an investigation?

Often, these terms are used interchangeably, but strictly speaking, “investigation” (or as is more generally used in the literature, “study”)  is the broader term. The US NIH defines a study like this:

“A clinical study involves research using human volunteers (also called participants) that is intended to add to medical knowledge. There are two main types of clinical studies: clinical trials (also called interventional studies [investigations]) and observational studies [investigations].”

So, clinical trials (or interventional studies) are a subset of clinical studies [investigations]where:

“Participants receive specific interventions according to the research plan or protocol created by the investigators. These interventions may be medical products, such as drugs or devices; procedures; or changes to participants’ behaviour, such as diet. Clinical trials may compare a new medical approach to a standard one that is already available, to a placebo that contains no active ingredients (or a sham device), or to no intervention. Some clinical trials compare interventions that are already available to each other. When a new product or approach is being studied, it is not usually known whether it will be helpful, harmful, or no different than available alternatives (including no intervention). The investigators try to determine the safety and efficacy of the intervention by measuring certain outcomes in the participants.”

In contrast, in an Observational Study [investigation]…

“…investigators assess health outcomes in groups of participants according to a research plan or protocol. Participants may receive interventions (which can include medical products such as drugs or devices) or procedures as part of their routine medical care, but participants are not assigned to specific interventions by the investigator (as in a clinical trial).”