IMMIGRATION POLICY AND THE AGRICULTURAL LABOR MARKET: THE EFFECT ON JOB DURATION

^σε

Also note that μ_-1 = -∞, μ₃ = ∞ from the assumption of normal distribution. In the
second stage of this Heckman type two-stage method, we estimate equation (2) below by ordinary

least squares with only completed spells.

^{ln t} = z ^{β +} P^σ j^λij ^{+ e} = z ^{β + β}_j^λij ⁺ eij

We can also show that the conditional variance of the log of the duration would be

var[ln t. | ln t. is observed] = ^σ_jI^{1 -} P ^{ββ ],}

where

• ʌ

^{- χ} α

^σε   J

_

= ∑ [ei² + /^λ δ_l^ ]/n_j. We can obtain the
i=1

л    Λ

estimator of the asymptotic covariance matrix for [β_j, β_λ ] by substituting these results in the
^{j j}

formulation in Greene (2003).

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