A negative control has to be able to fail
A placebo test condemned a mediation result, until a positive control showed it condemned everything.
- Topic
- Causal inference
- Analysis
- Aug 2026
- Status
- Research in progress
- Related
- CFPS research
In a longitudinal mediation analysis, a temporal-placebo negative control first suggested that an apparent mediation effect was an artefact. A positive control, the same test applied to the exposure–outcome effect we had most reason to believe, condemned that too. The control could not tell a real effect from a spurious one in this setting, so its verdict was withdrawn and the question moved to other tools.
The lesson is general: before a falsification test is allowed to reject a result, show that it can pass one.