I have the monthly indexes taken from the National Institute of Statistics of my country (below), then I can calculate its inter-annual variation easily:
((index month year - index same month year - 1) / index same month year - 1) * 100
But my problem is that I want to compare this inter-annual variation against Financial Statements that show 3, 6, 9 and 12 months values. Then this inter-annual variation is useless because it shows the price variation only for a month and not for 3, 6, 9, 12 months period.
To solve this problem, should I calculate the cumulative inflation of 3, 6, 9 and 12 months of each year, then calculate de inter-annual variation of that cumulative inflation and finally be able to compare them to the Financial Statement inter-annual variation?
Is there any other way to do this more accurately ?