In addition to the alternative time-series constructions, you can also define a time series with a numeric formula, using operators and functions to combine numbers and other time-series data. When a time series is determined by an arithmetic formula, the usual parameterisation is replaced by a read-only table of values for each year of the model run, which you can subsequently convert to an Interpolated Series if you wish to modify individual values – see 4.7 Viewing and editing time series.