covariance for stratified sampling

例句：

1. 1. The data shows a strong positive covariance between the two variables.
   数据显示这两个变量之间存在很强的正相关关系。
2. 2. In statistics, the covariance is used to measure how much one variable changes when another changes.
   在统计学中，协方差用于衡量一个变量在另一个变量发生变化时的变化程度。
3. 3. The researchers found that there was a significant covariance between the two variables.
   研究人员发现这两个变量之间存在显著的相关性。