non-experimental study, observational study,
study, non-randomized, also referred to as observational or non-experimental studies, do not use random allocation to assign participants to treatment comparison groups.
It is difficult to ensure that treatment comparison groups are sufficiently similar in terms of prognostic variables in non-randomized studies, particularly prognostic variables that remain undiscovered.
There is thus a greater risk of allocation bias in non-randomized studies than there is in randomized studies.
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