The Investment Simulation Spreadsheet is a simulation of saving and investing for retirement. It shows how much you can accumulate in a tax-deferred retirement account (e.g. an IRA or 401k account) over a 35-year period by saving a certain amount each year and investing it in a combination of fixed-interest or variable (equity) instruments.
The Income Simulation Spreadsheet shows how much income you can withdraw from a retirement account (e.g. an IRA or 401k account) that is invested it in a combination of fixed-interest or variable (equity) instruments.
Both of the simulations use a random-number
generator to simulate the fluctuation (volatility) in the
investment returns of equities (stocks and stock mutual funds).
Most spreadsheets have only a uniformly-distributed
random number function (RAND) and not a normally-distributed
random number function like a haystack curve, but it's much more
realistic to simulate deviations that are normally
distributed, because normal distributions have more small
deviations that are close to the mean and few deviations that are far from the mean. In
terms of investments, small losses and gains are much more
common that large ones. So these spreadsheets make use of the Central
Limit Theorem to create approximately normally distributed
random numbers by combining several RAND functions. For example,
the expression sqrt(3)*(RAND()-RAND()+RAND()-RAND()) creates nearly
normal random numbers with a mean of zero, a standard deviation
very close to 1, and a maximum range of ±4. (With 6 RAND
functions, the formulation works similarly, but has a larger
maximum range: sqrt(2)*(RAND(size(x))-RAND(size(x))+RAND(size(x))-RAND(size(x))+RAND(size(x))-RAND(size(x)))).