I-Orp vs. Cfiresim.com
when I run I-orp with all the same data (at least, data that is common to the two calculators) as cfiresim, I-orp yields an annual expenditure level about $9K higher than what cfiresim provides (about 66K instead of 57K (when using the 95% confidence level in cfiresim).
I entered into I-orp what I thought were modest numbers for stock returns, bond returns, and inflation, namely: 6%, 2.5% and, 2%, respectively.
Cfiresim uses aftcasting with historical market data. If the I-orp Monte Carlo option is not selected (I did not use this option in I-orp), perhaps I-orp uses some sort of constant return for the stock and bond markets? If true, this would explain the difference, as the 95% confidence level on Cfiresim allows for the possibility of some pretty low stock market returns, presumably lower than 6%. At least, that's my best guess so far.
However, if any of the learned members of this board have a better explanation, I would love to hear it. Unless I learn otherwise, I will stick with the more cautious approach of cfiresim, though I'd love to believe the more optimistic number that emerged from I-orp.