[tobias2014]

"Tracking ratings variance by release year, we observe an increase in review variability starting in the late 1970s, followed by a precipitous decline in variance starting in the early 2000s."

I'm sure there is also some time dependent effect in how people rate movies now vs. in the past. For a snapshot in current time, one would need to be careful to not include ratings from too long ago as society and norms change.