I use the VG Nest Egg Calculator. No matter what I put in, it always gives a 99% success rate. There's always that 1% chance of failure. Then, click projected savings balance, it's more reassuring. There's an explanation of how the success rate is determined. Shows graphs and historical data. I guess CD's give you one interest rate. They don't waver with market fluctuations.
"Monte Carlo simulation gives you a more realistic assessment of how the future may unfold by looking at a wide variety of potential market scenarios that take fluctuating market returns into account. Instead of basing calculations on just one average rate of return, we generate 100,000 simulations of hypothetical market scenarios, and calculate the impact on your savings during your retirement. Each simulation includes up and down markets of various lengths, intensities, and combinations."