What is Heteroskedasticity Recall the assumption of homoskedastic implied that conditional on the explanator variables the variance of the unobserved error u was constant If this is not true that is if the variance of u is different for different values of thex's. then the errors are heteroskedastic

Definition (1) Econometrics: economic measurement. (2) Econometrics: the social science in which the tools of economic theory, mathematics, and statistical inference are applied to the analysis of economic phenomena. (3) Econometrics: the result of a certain outlook on the role of economics, consists of the application of mathematical statistics to economic data to lend

Time series vs Cross sectional e Time series data has a temporal ordering unlike cross-section data Will need to alter some of our assumptions to take into account that we no longer have a random sample of individuals Instead. we have one realization of a stochastic(i.e. random) process Economics 20- Prof anderson

Testing for AR(IS eria Correlation Want to be able to test for whether the errors are serially correlated or not Want to test the null thatp=0 in u,=pu, 1 +et=2.. where u is the model error

Simultaneity Simultaneity is a specific type of endogeneity problem in which the explanatory variable is jointly determined with the dependent variable 2 As with other types of endogeneity, IV estimation can solve the problem o Some special issues to consider with simultaneous equations modelS(SEM) Economics 20- Prof anderson

一、二元离散选择模型的经济背景 二、二元离散选择模型 三、二元 Probit离散选择模型及其参数估计 四、二元 Logit离散选择模型及其参数估计 五、二元离散选择模型的变量显著性检验

• 一、多重共线性的概念 • 二、实际经济问题中的多重共线性 • 三、多重共线性的后果 • 四、多重共线性的检验 • 五、克服多重共线性的方法 • 六、案例 • *七、分部回归与多重共线性

一、多重共线性 对于模型 在求最小二乘估计时,要求XX的逆存在。当XX的逆不 存在时,即,x之间存在高相关的情况,我们称之为多重 共线性

Testing for Unit roots Consider an AR(1): y=a+p +e,t Let Ho: p=1, (assume there is a unit root) Define 0=p-1 and subtract y, from both sides to obtain Ay,=a+ B+e, Unfortunately, a simple t-test is inappropriate, since this is an I(1) process ADickey-Fuller Test uses the t-statistic, but different critical values Economics 20- Prof anderson

Choosing a Topic Start with a general area or set of questions Make sure you are interested in the topic Use on-line services such as EconLit to investigate past work on this topic Narrow down your topic to a specific question or issue to be investigated Work through the theoretical issue Economics 20-Prof. Anderson