Understanding published statistics.

Listening to “More or Less” on Radio 4 the other day, here are the items I remember..

1.What does the statistics “Ebola is up to 90% fatal” mean?

The meaning is based on CFR (Case Fatality Rate) – the ratio of reported cases to those reported cases which are fatal. The 90% figure came from a single outbreak in the Congo.

The CFR average all outbreaks since 1976 is between 60-65%.

The CFR for the current outbreak has dropped from 54% to 52% – a bit different that 90%!

2. What does the statistics that “a quarter of guide dogs working in London had been hit by a bike” mean?

The actual numbers come from a survey done for London and of 33 guide dog owners in London who responded, 14 said they had been involved in a collision. First the sample size is very small, second the person who came up with the number admitted it might be wrong as they were not good with maths!

The program reported that there are 320 registered people with guide dogs in London and all we can say is that 14 of those have been hit i.e. at least 4%!


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