Specific recommendations on techniques vary considerably, although a range of these are generally considered effective, and specific research on their comparative effectiveness is lacking. No single approach may be universally effective, either across learners or for the same learner across time, and trainers may need to adjust their techniques according to what is most effective in their situation. Training may begin shortly after birth in some cultures. However, in much of the developed world this occurs between the age of 18 months and two years, with the majority of children fully trained by age four, although many children may still experience occasional accidents.