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A factorial trial can assess several interventions within one structure. If the effect of one depends on the presence of another, interpretation of a main-effect summary changes. Efficiency does not automatically establish that the scientific questions are completely independent of each other.
Define the question and assumptions
Define questions for each intervention and combination. Explain conditions under which a main effect represents an average comparison. Consider plausible interaction, combination safety and available information during planning. Insufficient information to detect interaction is not evidence that interaction is certainly absent or that every pooled main-effect interpretation is appropriate.
Hypothetical study example
In a hypothetical trial, two interventions show one pattern separately and another when combined. The team does not present only a single main-effect summary pooled across all cells. It displays combination results and uncertainty, clarifying which assumptions support the planned interpretation and which observations might challenge those assumptions.
Design review checks
- Define intervention-specific and combination questions separately in the study protocol.
- Show participant numbers and endpoint availability within every factorial cell.
- Avoid interpreting interaction solely through separate significance labels across groups.
Record the decision and its limits
Accepted reporting distinguishes roles of main effects, combinations and interaction assessments. Address multiplicity and incomplete cells within the planned framework. Unexpected patterns may generate hypotheses, but should not be presented as prospectively confirmed by inventing a new hierarchy afterwards. Clinical safety decisions remain distinct from statistical convenience, with remaining uncertainty reported explicitly.
Frequently asked questions
- Is a factorial design two independent studies?
- Not exactly. Shared structure and interaction assumptions affect interpretation.
- Does nonsignificant interaction establish none exists?
- No. Available information and precision may be limited.