1.4. Why Are These Methods Called "Fixed Effects"?
The name "fixed effects" is a source of considerable confusion. As we shall see, the basic idea is very simple. Consider the linear model
where the i subscript refers to different persons and j refers to different measurements within persons—i.e., the same variable measured at different points in time. In conventional linear model terminology, β1 xij is described as a fixed effect because the xij terms are all measured values and β1 is a fixed parameter. On the other hand, εij is regarded as a random variable with a probability distribution, and we make certain assumptions about this distribution. ...
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