The trial was a logical extension of preceding observational and cross-sectional studies on estrogen replacement therapy. Overall, the studies suggested a 50 percent reduction in risk of coronary heart disease in current estrogen users compared to non-users. In spite of such striking findings, most studies had been prone to a number of biases. One major criticism of observational studies had been that women receiving estrogen were generally healthier and more compliant than non-estrogen users.