co-vary

1. The method is used when the model is complex and nonlinear ( that is, parameters do not co-vary in a linear manner), or when the model involves more than just two or three uncertain parameters.
   当模型复杂且非线性时（换句话说，在线性方式下参数不共变），或者当模型涉及多于两三个不确定参数时，使用该方法。
2. Many of the individual characteristics measured in risk studies co-vary with other individual differences.
   风险研究中测量的很多个体特征和其它的个体差别一起变动。

verb

1. vary in the same time period (of two random variables)