I'm putting some recent examples of probabilities together for our new intake of students and looked up coverage of a recent conjoined twins news story in the UK. The numbers as reported are baffling. Does anyone know where the much-quoted figure comes from: "The chance of survival is just 1 in 10 million"?
On 18 September the BBC said "The chances of surviving the rare condition are put at one in 10 million. Conjoined twins are very rare - only one in every 2.5 million births - and only 5% of conjoined twins are craniopagus, which means they are fused at the head. About 40% of twins fused at the head are stillborn or die during labour and a third die within 24 hours."
The next day they followed up with this: "Even before the operation the twins had defeated the odds: craniopagus affects one in 10 million births, approximately 40% percent are stillborn or die during labour, a third die within 24-hours, while just 25% survive."
Hmmm. I'm assuming there is much journalistic confusion going on here. But then I found the press release from Facing The World said this "Of all twins born (3%), 1:40,000 are conjoined and 5% of these are Craniopagus - joined at their heads, with fused skulls. Approximately 40 percent are stillborn or die during labour, one third die within 24-hours. Just 25 percent of twins survive (usually quoted as 1:10 million) and even fewer are able to undergo surgical separation."
None of which adds up to 1 in 10 million in any formulation unless my maths is seriously failing me. Anybody know where it comes from?
Senior Research Fellow in Quantitative Methods
Faculty of Health & Social Care Sciences,
St. George's, University of London & Kingston University,
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