Density and measures of  productivity may be simultaneously determined. This could happen because more productive places tend to attract more workers and as a result become denser. An alternative explanation, albeit equivalent from an econometric perspective, is that there may be a missing local variable that is correlated with both density and productivity. We refer to this issue as the “endogenous quantity of  labor” problem.

请问一下：红颜色的是什么意思？

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