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q_{i,t}=S_{i,t}−S_{i,t−1}−R_{i,t}+d_{i,t−1}= \widehat {D} ^{L_i}_{i,t}−\widehat{D}^{L_i}_{i,t−1})−R_{i,t}+z(\widehat{σ}^{L}_{i,t}−\widehat{σ}^{L}_{i,t−1})+d_{i,t−1}