Evidence-appraisal glossary
Factorial trial
A randomized trial that tests two or more interventions at once by randomizing participants to every combination of them, letting a single trial answer more than one question.
Also called: factorial design.
In a two-by-two factorial trial, participants are randomized independently to intervention A or not and to intervention B or not, so each treatment is evaluated across the whole sample. This is efficient when the treatments are expected to act independently. The key assumption is that the treatments do not interact; if the effect of A depends on whether a person also got B, the main analyses can mislead, and detecting that interaction reliably usually needs a much larger sample than testing either treatment alone.
This is a plain-language methodology definition for reading research. It is general education, not medical advice.