A price index defined as the weighted arithmetic average of the current to previous period price relatives using the values of the base period as weights. The price reference-period is the previous period (that is the period immediately before the current period) and the weight reference-period is some other fixed period prior to the previous period. A time-series index is derived by chaining which, because the weight reference-period remains fixed, can result in a serious upward drift in the index when price changes are large and erratic.