Revealed comp. advantage:
M_{c,p} = 1 if RCA_{c,p} ≥ 1
Fitness (Tacchella 2012):
F̃_c = Σ_p M_{c,p} · Q_p
Q̃_p = 1/(Σ_c M_{c,p}/F_c)
Normalize: F_c = F̃_c/⟨F̃⟩
ECI (Hidalgo):
ECI = (k_c − ⟨k_c⟩)/σ
Fitness = nonlinear iteration
captures nested structure of trade