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Matrix predictors described in Eq. 51 combine in a linear manner to form the fixed-effect matrix featured within the mixed-effects regression model Eq. 50. Air traffic proximities (A) are proportional to the number of air passengers exchanged between airports within respective regions (3). Intracontinental proximities (B) take values between 1 for regions on different continents to 1 for adjacent regions. The Hubei asymmetry (C) roughly characterizes the Hubei quarantine of early 2020.
