Compare adjacent periods more carefully
Economic activity often has recurring calendar patterns: hiring around holidays, seasonal construction and predictable shopping cycles. Seasonal adjustment estimates these patterns and removes their effect from a series to make adjacent periods easier to compare. It does not remove every irregular event or guarantee that the remaining movement has one cause.
An unadjusted value is the observation under the program’s original publication basis. An adjusted value incorporates estimated seasonal factors and sometimes other calendar treatment. Both are useful, but they answer different questions and should remain separately labeled.
Keep the basis consistent
A monthly change should use the same adjustment basis at both endpoints. Do not divide a seasonally adjusted current index by an unadjusted previous index. The resulting ratio mixes measurement conventions rather than isolating the period’s price change. This site stores CPI adjusted and unadjusted inputs as distinct variants.
Twelve-month comparisons often use unadjusted values because the same month’s seasonal position is compared across years. This is not a universal rule: a program may publish adjusted yearly growth too. Read the exact source definition rather than assuming that every headline uses the same convention.
Adjusted histories can change
Seasonal factors are estimates. Re-estimation with more observations can change historical adjusted values even when the unadjusted source observation has not changed. A revision to an old period must be distinguished from an economic change between two observation periods.
A method or population change may create another break in comparability. Household-survey population controls, establishment benchmarks and industry classifications require source-specific notes. Seasonal adjustment alone cannot make differently defined populations or industries identical. A smooth-looking chart is not proof of an uninterrupted measurement history.
Record what you used
A defensible citation includes the series, frequency, adjustment, reference period, transformation and vintage or acquired snapshot. When a producer changes an adjusted history, the archived site file retains the earlier acquired version. The first site snapshot does not supply authentic vintages from periods before acquisition.