N trend measures the absolute difference between a period N and N-1, N-1 being here defined as the adjacent period.
Depending on the analyzes, the comparison period will be
Weekly: the previous week (s37 vs. s36)
Monthly: last month (September 2017 vs. August 2017)
Quarterly: the previous quarter (July 2017 to September 2017) vs. (April 2017 to June 2017)
Annual: previous year (2017 vs. 2016)