計量的causality - 經濟

出處:http://tinyurl.com/ykbawo6

reg lines from economic data often cannot be given a causal interpretation.

The reason being that in the relation of interest between observables

and unobservables we might expect they are correlated, whereas in a
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reg model regressors and unobservables are uncorrelated by construction.

請問黃字文意為何?

y=xb+e

迴歸假設之一： E[xe]=0 為了得到b的認定.

不懂的是為何 E[xe]!=0 會是expect的條件為了casuality的關係？

謝謝^^"

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