第一问答案没理解

问题如下：

What conditions are required for a regression to suffer from omitted variable bias?

选项：

解释：

There are two conditions. First, the variable must be omitted in the sense that it has a non-zero coefficient in the correct model. Second, the omitted variable must be correlated with an included variable. If the X variables are uncorrelated, then the coefficients can be estimated consistently even if there is an omitted variable.