An Interpolated Series is a special case as you can effectively vary the number of parameters by adding fixed values for specific additional periods, or by removing them. Each fixed value is displayed in black, while the calculated interpolated values are shown in grey.
Suppose we defined penetration for a Service as an Interpolated Series with values 0.5 in Y0 and 0.6 in Y5. (This is most familiarly achieved by accessing the parameters dialog, selecting Interpolated Series from the Type menu, and then entering values for these two years.)
Figure 1: Penetration defined as an Interpolated Series
Note: Values within an Interpolated Series cannot be defined by a formula.
The fixed values can also be modified directly in the time-series view. The following sections explain how interpolated values in a time-series view can be modified.
Adding another fixed point
If you type a new value for a calculated cell, e.g., 0.59 in the Y3 column of the time-series view and press <Enter>, the Editor asks if you want to adjust the surrounding calculated values. By default, the Y3 value is fixed and the intermediate values for Y1, Y2 and also Y4 are re-interpolated to meet this point. This is equivalent to adding a single node in the separate Interpolated Series dialog.
Figure 2: Adjusting surrounding values between Y0 and Y5
Otherwise, if you select No, then STEM freezes the current values for the adjacent periods in order to preserve the other interpolated values. In both cases, the scope of the change is limited to the interpolation interval containing the modified value.
Figure 3: Freezing the adjacent values
Removing a fixed point
If you subsequently decide to reduce the number of data points or nodes:
- Select the fixed value you wish to discard with the mouse or cursor keys.
- Select Unset from the dialog Edit menu, or press <Delete> to delete the value text from the formula bar and then press <Enter>. The fixed value is removed and the neighbouring values are re-calculated to fit the next adjacent fixed values.
Note: You can remove several fixed points at once by pressing <Shift> to extend the selection of cells and then selecting Unset from the dialog Edit menu.