I have data that consists of dates and monthly CPI belong to that date. And I'm trying to find Inflation belongs to specific date, for instance inflation at 2015-02.
I used this formula to calculate for example inflation rate in 2016-01
((i/l)-1)*100 = inflation rate i = CPI value of the current month l = CPI value from 12 month ago
( (-1,90 / -1,60) - 1 ) * 100 = 18,75
But it doesn't make sense because result of formula is 18,75 but the real inflation rate at 2016-01 is different.
Also, for some other month I found -73,68 value from that formula, It also doesn't make sense at all.
How can i find monthly inflation according to specific CPI value belongs to that month?