Increasingly, online retirement income calculators are abandoning "deterministic gap analysis" as a way to provide a comprehensive range of retirement income probabilities for 401(k) participants. Instead, Monte Carlo simulation has become the hot methodology. Unlike deterministic gap analysis, which requires users to forecast average rates of inflation and investment return, Monte Carlo randomly generates possible values for key variables such as interest rates, inflation, and investment returns. Financial planners like Gary Cohen, national solution leader for eAdvisor Plus, an online retirement planning product from Ernst & Young, says Monte Carlo gives a more accurate reading than deterministic gap analysis. "The inherent weakness of that [deterministic gap analysis] approach is you're ignoring all the volatility that occurs," Cohen says. As with most trends, there is always the risk of misapplication. Chris Jones, executive vice president of financial research and strategy at Financial Engines, warns that there is wide variation among Monte Carlo models, and if not carefully applied, a person could end up generating unrealistic scenarios that are not typical of situations that may occur.