Multi-stage Model Y = LF, t = age, Z = pollution 1: Y cij = a ci + b ci t cij +  1 Z ck  s(t cij ) + e cij – b ci = subject-specific 8-yr LF growth 2: b ci = B c +  2 Z ci + e ci – Regression on subject-specific variables 3: B c =  0 +  3 Z c + e c – Regression on ambient pollution level Fit as single mixed model Can include confounders at each level Berhane et al, Statist Sci 2004; 19: 414-440
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