With an even number of years (4), there is no single middle year, so the "$2X$" coding (-3,-1,1,3) is used, with each unit of $X$ equal to half a year. …

Step 1 — set up the coding table using the "$2X$" method (since $n=4$ is even, there is no single middle year; the two central years 2020 and 2021 are coded $-1$ and $+1$, and the outer years $-3$ and $+3$, so each unit of $X$ represents half a year):

Step 2 — compute $a$ and $b$:

Trend equation: $Y_c = 50 + 3.6X$, origin at the midpoint of 2020 and 2021, with each unit of $X$ equal to half a year.

Step 3 — trend values:

Using consecutive integers $-2,-1,1,2$ (a gap of 1 between the two central values) instead of $-3,-1,1,3$ for the 2X method — the whole point of the 2X coding is that every value differs from its neighb …