An item in Think Tank on April 27 erroneously made it appear that a study had found sterilization of women a far less reliable birth control method than it is. The item quoted a failure rate of 8.2 per hundred, when research by Dr. Icheng Chi and others found a rate of 8.2 per thousand. Said Dr. Chi: "Nothing in medicine is foolproof, but clearly voluntary sterilization is an excellent method and deservedly widely used."

Surgical sterilization for women has become the most popular form of birth control in the country, and one of the reasons is that it is promoted as 100 percent effective.

But a new study suggests that not all surgical techniques employed are that effective and that, overall, sterilization carries a higher risk of pregnancy than the pill. The pregnancy rate for sterilization varies from 0 to 8.2 per 100 women per year, depending on the tecnique, while the rate associated with the pill is 0.7 per 100. (For the IUD, it is 2.6 per 100.)

"Surgical sterilization is not foolproof," warns Dr. I-Cheng Chi of the International Fertility Research Program in Research Triangle Park, N. C., who reviewed the records of 14,700 sterilized women. Chi and his colleagues, researchers from Duke University, reported their results in the American Journal of Obstetrics and Gynecology.

"The procedures used in the 14,700 sterilizations involved cutting, burning or pinching off the fallopian tubes, which carry the egg from the ovaries to the womb. The primary reason for failure is that somehow enough of a passage remains for the sperm to get by and fertilize an egg. This can sometimes result in an ectopic, or tubal pregnancy which can threaten the woman's life, Chi said.

The least effective surgical technique involves a vaginal entry to reach the fallopian tubes, which are then pinched closed with a tantalum clip, Chi said.Another uncertain technique involves an obdominal incision through which a spring clip is placed on the tubes.

Burning the tubes apart with a hot instrument and pinching them off with a falope ring that looks like a rubber band are highly effective measures, Chi reported.