On the news yesterday there was this guest who just kept saying "incidences" when describing the frequency of an event (e.g. there were 15 incidences of teenage stabbings in London in the first 6 months of 2010). Surely she should be saying incidents. Does incidences exist as a word and if so what is it's correct use?
She wasn't using it correctly. I we all agree that "incidence" means "frequency", try putting that into what the speaker said, viz; "there were 15 frequencies of teenage stabbings in London in the first 6 months of 2010". Something you might hear in a maths lesson, but not what was meant in this context, IMHO.