(I) At present, condoms and pills are the most common forms of birth control used in the Western world. (II) These are 98 and 99 per cent effective, respectively. (Ill) Some other methods are long-acting contraceptives, such as subcutaneous implants or injections. (IV) These keep working for anything from two months to five years and can achieve reliabilities in excess of 99 per cent (V) The reason was that roughly one in 200 female tubal sterilizations and one in 2,000 male vasectomies are unsuccessful.