The book where the original 1 in 4 figure comes from

Mental illness in the community: the pathway to psychiatric care By David P. Goldberg, Peter Huxley
http://books.google.co.uk/books?hl=en&lr=&id=yU0OAAAAQAAJ&oi=fnd&pg=PR7&dq=undiagnosed+rate+incidence+mental+disorder&ots=9-mcNMuHgA&sig=xW8t5NkRDylS_GDS8vb6jGuJOPc#v=onepage&q&f=false


It's somewhere in here they estimate a 250 in 1000 incidence figure from a 180 in 1000 prevalence figure. I think the sample period was a month for the prevalence figure.

However definitions of mental illness are important. I haven't read the book thoroughly to see which measure was used to calculate the prevalence figure. The calculation that goes between the prevalence and incidence was noted to be incorrect in the authors second book published in the nineties.