Just one month after a study found that women who took hormone replacement therapy after menopause had an increased risk of breast cancer, another study has found no such effect.

The new study, being published today in The Journal of the American Medical Association, compared some 500 women aged 50 to 64 who had newly diagnosed breast cancer with a similar group of healthy women. The authors found no link between use of the hormones and breast cancer. In fact, they found that women who had used hormones for eight years or longer had, if anything, a lower risk of breast cancer than those who had never taken them.

The latest development, scientists say, is good news tinged with great uncertainty.

The previous study, published on June 15 in The New England Journal of Medicine, followed 122,000 nurses for 14 years and found that hormone replacement therapy increased the risk of breast cancer by 30 to 70 percent.

Researchers say the conflicting results of the two highly regarded studies underscore the fact that this is science in progress: they are nowhere near obtaining a completed picture of the risks and benefits of taking hormone replacement therapy after menopause.

what are the differences in the design of these two studies? How might these differences contribute to different outcomes?