Why it matters in the Letby case
The defence and post-conviction statistical critique argue the Letby case did not present the jury with the comparator base rate of nurse-shift presence at adverse events in similar units.
Statistical explainer
Base rates in healthcare clusters
A base rate is the frequency of an event in the relevant comparator population. Without a base rate, 'she was there' has no inferential weight.
Statistical analysis2Expert/professional
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Prosecution position
The Crown's presentation rested on Letby's presence at every charted event without setting that against how often any nurse would be present at adverse events on a comparable unit.
Expert challenge / post-conviction reading
Post-conviction statistical critique argues the jury was never given the comparator base rate. Without it, 'she was there' carries no inferential weight, because the expected frequency of coincidental presence is unknown.
What remains uncertain
A base rate for this unit over this period would require an independently defined event set and complete rota data for all staff. The critique argues it was not produced; it has not been produced since either.
Common questions
What is a base rate?
The frequency of something in the relevant comparison population — here, how often a nurse would be on duty for adverse events on a unit like this one.
Why does the absence of one matter?
Because without it there is nothing to judge 'she was present at all of them' against. The pattern might be extraordinary or unremarkable, and the chart alone cannot say which.
Was a base rate presented at trial?
The defence and post-conviction critique argue it was not.
Can one be calculated now?
Not straightforwardly. It would need an event set defined by clinical criteria alone and full rota data for every member of staff.
Related reading
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