perusjakson

Perusjakso, or base period, is a fixed reference period used to measure relative changes in a time series, such as prices or quantities. It assigns a value (commonly 100) to all observed values relative to the base period, anchoring the construction of index numbers.

In statistics and economics, base periods are used to compute indices like consumer price indices (CPI), GDP deflators, or producer price indices. An observation in the current period is divided by the base-period value and multiplied by 100, producing a dimensionless index that makes changes over time comparable.

There are fixed-base indices, where the same base period is used throughout the series, and chain-linked indices, in which the base period is updated periodically and index values are re-referenced to maintain current relevance. Switching the base period may change the numerical values but not the underlying growth direction.

Choosing a base period involves trade-offs: it should be representative of typical conditions, long enough to smooth seasonal effects, and allow for historical comparability. Base periods are sometimes updated to reflect structural changes in the economy; such revisions require re-indexing to preserve continuity.