Why it matters in the Letby case
Defence and post-conviction commentary argue the prosecution's common-factor framing was inferentially weaker than its rhetorical force suggested.
Statistical explainer
Common-factor reasoning errors
Identifying a common factor across a set of curated events is trivially possible in any cluster, especially when the events were chosen because of the factor. Inferring cause requires more than association.
Statistical analysis2Expert/professional
Last updated
Prosecution position
Letby's presence was offered as the common factor across the charted events — the one thing they shared.
Expert challenge / post-conviction reading
Defence and post-conviction commentary argue that identifying a common factor across a curated set is trivially possible, particularly when the set was assembled because of that factor. Association is not cause, and the rhetorical force of 'the one common factor' outruns its inferential weight.
What remains uncertain
Other common factors across the same events — unit conditions, staffing, acuity, specific equipment or practices — have not been systematically tested against the same event set.
Common questions
Why is a common factor weak evidence on its own?
Because in any set of events there are many shared features, and if the set was chosen because of one of them, that feature is guaranteed to be present.
What other common factors existed?
The unit's staffing, acuity mismatch and physical condition across 2015-2016 are documented on the Thirlwall record. Whether they were tested against the same event set is a separate question.
Does this mean presence is irrelevant?
No. It means presence has to be assessed against how often it would occur anyway, which is the base-rate question.
Is this different from selection bias?
It is the consequence of it. Selection bias explains why the common factor was guaranteed to appear.
Related reading
Found an error or missing source? Send a correction with the page URL and supporting source.