|Introduction: The Time Spread History family is an extension of the Time Spread family that contains functions that can be adjusted for historical payments or payments made "to date", a problem often found in budgeting. The aim of these functions is to make the overall amount disbursed equal to the overall Total, adjusting the future rate of disbursement accordingly.|
There are two ways in which the payments to date are taken into account:
- One way is to explicitly specify a payments history using the variables DatesHistory and PmtsHistory. The remainder of the Total, after taking into account the historical payments, is spread according the different kind of spread mechanism in the function. The way these functions work is that firstly the historical payments are calculated to see if any historical payments occur in the timeperiod. Then the Total is reduced by the total amount of historical payments made to date, or payments yet to be made. Then the function tries to spread the remaining total in its usual way, either uniformly in the case of UniSpread,or in an S-Curve as in SCurve, etc. It will probably do this calculation internally more than once and adjust it, to make sure that no more or less than the Total is dispersed. To make it absolutely clear, the function will always disperse exactly the Total over the period of the projection, no more and no less.
- A second and more direct way is to specify the overall amount incurred prior to Time in a single ˙ActualsToDate˙ variable, for example in the UniSpreadAdj function. Here the Total is spread over the appropriate length of time after allowing for the ActualsToDate. This is not as trivial a calculation as first appears, because for functions like SCurveAdj the remainder of the curve has to honour the original shape before the actuals were incurred. Indeed, the function arrives at the forecast by pro-rata-ing the original forecast so that the total forecast by the function, plus the ActualsToDate, always comes to the Total. See the example of the SCurveAdj function for a demonstration of this principle. Again, the function will always disperse exactly the Total over the period of the projection, no more and no less.
Note: These functions will disperse the Total, no less and no more, regardless of the amount of historical payments specified.
These functions are very good for budgeting, where you always have a projection for the future and are constantly updating the past data. Without this family of functions it is a pain to keep changing your forecast to reflect actuals, whereas these functions do that job for you.