Seasonal adjustment
Removing regular calendar patterns so month-to-month changes can be compared.
2 min read · Reviewed September 2026
In plain words
Seasonal adjustment removes predictable calendar effects — cheaper flights in September, higher food prices at Christmas — so you can tell whether this month's move is real or just seasonal. The 12-month rate is naturally seasonal-adjustment-free; MoM rates are not.
Frequently asked questions
Why is some CPI data seasonally adjusted?
Prices have predictable patterns — cheaper flights in autumn, higher energy bills in winter. Seasonal adjustment strips these out so you can tell whether a month-on-month move reflects real price pressure or just the calendar.