A base year refers to the first of a years's series in an economic or financial index. New base years are introduced on a regular interval to keep data updated in a particular index. A base year helps to carry out comparisons in the measure of a business activity or economic index.
Trend analysis is the term used for calculating the amount of the increase/decrease for the period by subtracting the earlier year from the later year.
Negative difference means the change is a decrease, and positive difference represents an increase. So, in the context of trend analysis, the base year will be the current year.
