Step 1: When two index number series with different base periods overlap and need to be joined into one continuous series, the technique used is called splicing.
Step 2: Splicing has two forms depending on which series is adjusted onto which base. If the figures of the old series are converted and connected onto the base of the new series, so that the old series is carried forward and made consistent with the new one, this is called forward splicing.
Step 3: If instead the new series is converted back onto the base of the old series to extend the old series with fresh data, that direction is called backward splicing. Base shifting, by contrast, simply changes the reference base of a single series without joining it to another series with a different base, so it does not match this description.
Step 4: Since the question describes connecting the old series onto the new series, the correct term is forward splicing.