basejaar

Basejaar (base year) is a reference year used in the construction of index numbers to compare economic quantities over time. In most price or quantity indices, the base year is assigned a value of 100 (or 1), and all other years are expressed relative to that reference point.

To compute an index for year t, divide the observed value in year t by the value in the base year, then multiply by the index base (commonly 100). This produces a time series that shows the percentage change since the base year.

Important considerations: the choice of base year affects the level of the index, and changes in weighting or market patterns over time can make a previously computed index less representative. Therefore indices are periodically rebased to a more recent year, ensuring the weights reflect current patterns while preserving comparability of growth rates through chain-linking or consistent rebase.

Base year decisions revolve around representativeness. A fixed base uses the same weights forever; a chained (or chain-linked) index uses successive base years to reduce distortion from shifting consumption patterns.

In official statistics, agencies such as the Dutch Centraal Bureau voor de Statistiek (CBS) publish price, expenditure, and production indices with specified base years and rebasing schedules. The base year is a methodological tool rather than an intrinsic economic quantity.