In many dynamical systems, there is a large separation of time scales between typical events and "rare" events which can be the cases of interest. Rare-event rates are quite difficult to compute numerically, but they are of considerable practical importance in many fields, for example, transition times in chemical physics and extinction times in epidemiology can be very long, but are quite impo...

OBJECTIVE To assess effects on condom use and other sexual behaviour of an HIV prevention programme at school that promotes the use of condoms with and without emergency contraception. DESIGN Cluster randomised controlled trial. SETTING 40 public high schools in the state of Morelos, Mexico. PARTICIPANTS 10 954 first year high school students. INTERVENTION Schools were randomised to one...

Objective: To assess whether provision of educational leaflets or questions on contraception improves knowledge of contraception in women taking the combined contraceptive pill. Design: Randomisation of women into three groups according to type of educational leaflet on contraceptive information. These groups were subdivided into two on the basis of questions on contraception asked by the docto...