STEM help / Editor skills

4.7.3 Time-series formulae

In addition to the alternative time-series constructions, you can also enter numeric formulae on the button for the time series as a whole. A simple numeric constant, x, is taken as a short cut for setting Constant = x ‘inside’. Any other formula is evaluated as an explicit time series: the usual parameterisation inside is replaced by a read-only table of values for each year of the model run. You can use all the usual mathematical operators and functions, as well as some special time-series operators.

For example, you could define a time series to be some function of another time series:

  1. Enter the formula in the time-series field.
  2. Click on the other time-series field to generate the corresponding reference text. (This procedure is described in detail in 4.12 Creating formulae and internal references.)

If the other series was defined as, say, an Exponential Growth, STEM would temporarily evaluate the Exponential Growth in order to produce the explicit values for the calculated series.

An explicit time series defined by a formula

An explicit time series defined by a formula can be converted into an Interpolated Series by freezing the formula. Either:

  • select Freeze from the Edit menu on the dialog with the time-series button, or
  • select Freeze from the Edit menu on the inner, time-series dialog.

Operators and functions

STEM supports many familiar (and a few not-so-familiar) operators and functions which you can use in formulae for numeric inputs. Precedence and associativity of binary operators can be imposed with parentheses in the usual way.

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