同时方程模型：
到目前为止,我们学习计量经济学模型的助教lk about single equation regression models where a dependent variable is related to a set of independent or explanatory variables. In all these regression models, y is the 'dependent' or 'explained' variable arid x1，，，，x2，....是“独立”或“解释性”变量。因此，y取决于x，但反向不正确。但是，在现实生活中，我们通常会发现彼此依赖的变量。在这种关系中，可以通过考虑关系中的同时性来确定变量的平衡值。
在同时关系中，所研究的单个方程是更广泛现象的一部分。在这种情况下，可以借助方程式来解释变量。让我们举个例子。从宏观经济学课程中知道，在宏观层面上，总共消费支出会以总收入的总收入为基础。另一方面，总可支配收入取决于政府施加的国民收入和taw。此外，国民收入取决于经济的总消费支出。
Thus estimation of a single equation between aggregate consumption and disposable income will not provide us with consistent and unbiased estimators.
Unlike the single equation model, in simultaneous equation ilodels we get more than one dependent, or endogenous variables. This endogenes variable may appear in one equation as endogenous and as exogenous in another equation of tne system. Due to presence of the endogenous variables as explanatory variable in some of the equations it becomes correlated with the disturbance term of the equatioh in which it appears as the explanatory variable.