For example, 𝜖𝜖𝑡𝑡 may be thought of as an event outside of Canada that leads to a repricing of longer-term mortgage rates. In general, 𝜖𝜖𝑡𝑡 should not be thought of as a mean zero random variable. In fact, it is often best to think of 𝜖𝜖𝑡𝑡 as a random variable that only takes on non-negative values.
(2")  𝐶𝐶𝑡𝑡 = 𝛼𝛼1 + 𝛼𝛼2 𝑌𝑌𝑡𝑡−1 + 𝛼𝛼3 (𝐷𝐷𝑡𝑡+1 − 𝐷𝐷𝑡𝑡 ) − 𝛼𝛼4 𝜖𝜖𝑡𝑡 ∗ 𝐷𝐷𝑡𝑡 − 𝛼𝛼5 𝜖𝜖𝑡𝑡  As long as 𝜖𝜖𝑡𝑡 = 0—that is, as long as the trigger is dormant—the negative effect of the vulnerability on activity is muted.