One in Eleven is a new Hearth & Horizon story (text only), set in an invented parish in Cape Breton in 2048. A health authority's assistant has to tell two parents what a positive newborn screen means. The arithmetic, with assumed figures (not any real programme's):
- Prevalence 1 in 10,000, sensitivity 99%, false-positive rate 0.1%.
- In 10,000 babies: 0.99 true positives and about 10.0 false ones. About 11 are flagged; one is real. The chance a flagged baby has the condition is 0.99 / (0.99 + 10.0), about 9%, not 99%.
- Counting natural frequencies ("a hall of ten thousand babies") is easier to follow than quoting percentages, and it is how the assistant explains it.
- The practical rule in the story is the same either way: act as though it could be real (regular feeds, a clear emergency threshold) while speaking as though it probably is not, until the second-tier gene test.
The story also lets a grandmother's hurry to baptise, and the priest's answer, stand on their own. Read it on the Stories page. It is fiction and not medical advice.