If You Included Both Time and Entity Fixed Effects in the Regression

question 11

Short Answer

If you included both time and entity fixed effects in the regression model which includes a constant, then a. one of the explanatory variables needs to be excluded to avoid perfect multicollinearity.
b. you can use the "before and after"' specification even for $T > 2$ .
c. you must exclude one of the entity binary variables and one of the time binary variables for the OLS estimator to exist.
d. the OLS estimator no longer exists.

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Definitions:

Maxilla

the upper jawbone in vertebrates, which is key to the formation of the palate, nose, and eye orbits.

Lumbar Curvature

The inward curve of the lumbar spine (lower back), which is a natural part of the spine's shape.

Sacral Curvature

The natural inward curvature (lordosis) of the sacral spine region that contributes to the overall S-shape of the spinal column.

Thoracic Curvature

The natural curve of the thoracic spine, which is convex in shape when viewed from the side.