
Originally Posted by
publius
The "fiscal gap" NPV is a complicated mess (and ultimately dependent on various assumptions about the future) actually, but can be broken down into simple pieces and we can sort of get the feel for it and that might be helpful.
At the most basic level, the PV of some amount A at some time t in the future (t = 0 is now), is simply "discounting it" by the expected interest rate r over that time t: PV = A/e^rt. Of course, that is just the elementary "time value of money" concept. If you put some PV amount of money away in the bank (or whatever investment) at interest rate, r, A is what it compounds to in the future (assuming the bank survives).
[Note to any lurkers concerned about using 'e' -- this generally just makes things simpler and slicker than using the discrete geometric form (1 + r) but they are otherwise equivalent, only difference is what 'r' is . Sometimes, though the geometric form does work out better, though. e is the limit of (1 + 1/n)^n as n goes to infinity, and that comes about from the derivatives, actually. When dA/dt/A is exactly equal to 1, then A = e^t, but I ramble...]
A zero coupon bond is the simplest example of this, of which form short term bills usually are done. The face value of the bill is A, and PV is the money raised now at the bill auction.
Anyway, given PV = A/e^rt, we see that PV decreases as t and r increase, and increases as they decrease. Thus the closer we get to D-day when we must pay A, the more we need now. And the lower the interest rate, the more we must put away. And if we start taking derivatives with respect to r and t and all that good stuff (note taking dervivatives of e^rt is very simple), we see that while PV decreases as t increase, it becomes more sensitive to changes in interest rate.
And then we can get fancy and let interest rate r itself vary with time, and e^rt becomes something more complicated. And involves some projection of the future.
Next complication is adding a series. Rather than one single lump payment at one single time in the future, we have a series of n payments, A_i, we make at a series of future times. Thus we sum up the PV for each term.
PV = A1/e^(r*t1) + A2/e^(r*t2) + ... A_n/e^(r*t_n) ...