Consider the population regression functions (PRF) below. For all function, we assume that the vertical intercept (a.k.a., the constant term) is equal to zero.
PRF1: uₜ = ɣ₁yₜ₋₁ + ɣ₂yₜ₋₂ + uₜ
PRF2: yₜ = ɣ₁yₜ₋₁ + ɣ₂yₜ₋₂ + αxt + βzₜ + uₜ
PRF3: yₜ = ɣ₁yₜ₋₁ + θ₁ uₜ₋₁ + uₜ
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Which one(s) of the above PRFs is (are) not a univariate model?
a. PRF2
b. PRF2 and PRF3
c. PRF1 and PRF3
d. PRF1 and PRF2
e. PRF3
f. PRF1