科学实验必须考虑到所有相关变量。

Scientific experiments must account for all relevant ___variables.

在心理学研究中，幸福感是一个复杂的变量。

In psychological studies, happiness is a complex ___variable.

经济预测包含几个关键变量，比如利率和就业率。

The economic forecast includes several key ___variables, like interest rates and employment.