Predictions with a purpose

摘要

The Ebola epidemic in west Africa appears to be nearing its end. The number of new cases per week has averaged around 120 for the past three weeks in Guinea, Liberia and Sierra Leone, the lowest level since July. A trial of a potential treatment has been halted for lack of patients. The last embers will be difficult to extinguish; the numbers ticked back up in the latest weekly data. But the World Health Organisation (who) is now focused, not on control, but on snuffing it out. With more than 22,000 people infected, nearly 9,000 of whom died, this out- break was the gravest ever. But it confounded experts who had feared much worse. In September the who predicted more than 20,000 cases by November (there were actually about 13,000). Around the same time, a worst-case prediction by America's Centres for Disease Control and Prevention (cdc) of up to 1.4m cases (reported and unreported) by January 20th made the headlines. Others were similarly bleak. So where did models and reality diverge?