Student or Learner
What are the differences in the sentence below in terms of the preposition of "in" and "by". And when should I use "in" and when "by" when describing a graph?
"This number decreased gradually to 500 by 1974 and continued to decrease but more steeply to 250 in 2000."
'by' indicates a deadline, or the end of a particular time period
e.g. "I've got to do this report by Monday."
In your sentence:
In 1966, there were 750 newly ordained priests. This number decreased gradually to 500 by 1974 (=1974 being the end of the time period from 1966 to 1974).
Apparently, over the period 1974 to 1999, the number continued to decrease, and in the year 2000, the number remaining (of whatever it was) was then 250.