linguistic:voice_change
Hausman–Taylor method

Estimating γ {\displaystyle \gamma } via OLS on d i ^ = Z i γ + φ i t {\displaystyle {\widehat {di}}=Z_{i}\gamma +\varphi _{it}} using X 1 {\displaystyle X_{1}} and Z 1 {\displaystyle Z_{1}} as instruments yields a consistent estimate.