Those who use Internet financial calculators to see if they will have a secure retirement know this feeling: Inputting your data on one site elicits a jubilant "yippee." Plug in the same data on another site and you're saying "yikes."

The variation in results can be attributed to different assumptions and defaults built into each calculator. Results can differ because calculators do not ask for the same information. Then there are the mistaken assumptions you may make when you input your information.

"The two biggest ones are life expectancy and investment returns," says Steve Vernon, a columnist with CBS MoneyWatch. Vernon says people often underestimate their longevity and overestimate their investment returns.

Vernon approaches retirement planning differently than financial advisers. An actuary by training, Vernon uses historical financial rate-of-return data and life-expectancy statistics to determine the risks to a secure retirement.

If you use a calculator, what should you input for your life expectancy? "A lot of people hear that the average life expectancy in America is in the late 70s," Vernon says. "They don't realize that is the average life expectancy from birth." Vernon notes that once you hit your 60s, you have a good chance to make it to your 80s and beyond. In fact, actuarial tables show a 65-year-old woman can expect to live until she is 85 and a 65-year-old man can expect to see his 82nd birthday.

As for rate of return on investments, Vernon believes many projections are too rosy. "You'll hear financial advisers say you can get 7, 8, 9 percent over a long period of time, but I think we need to ratchet down our rate-of-return assumptions," he says. "I advocate people look at 5 or 6 percent for a balanced portfolio."

Vernon, an author of several retirement books and head of corporate consulting firm Rest-of-Life Communications, analyzed seven free retirement calculators for a series of recent Moneywatch columns. While he kept his data for a typical couple consistent, the results were wildly divergent. For example, the SmartMoney calculator ( said the couple was on track for a secure retirement, but Charles Schwab's calculator ( said the couple might face a $2.6 million shortfall.

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Of the seven calculators Vernon analyzed, his two favorites were offered by AARP ( and MSN Money (