我在读一篇英文的药品对照疗效试验的前期声明中，有这样一段话：“Statistical Considerations A total of 600 patients will be randomized equally between the 2 arms. This will provide a minimum power of 80% to detect an HR of 0.74 at a 2-sided type-one error rate of 5%.” 其中英文缩写“HR”是什么意思？为什么要计算HR,怎样计算？请各位大侠帮忙，衷心感谢！