It's hard to plan for retirement, and one of the trickiest questions is: How much should I save?

The honest answer is: It depends. "The best-laid plans can be undone by a messy divorce, a disabling disease, or a stock market crash," Jonathan Skinner, a professor of economics at Dartmouth College, wrote in a study on the topic. Your future health-care expenses are almost impossible to predict, for example, especially as Congress considers big changes to the system.

Aug. 26 (Bloomberg) -- Medicare continues to exhibit remarkably slow growth: a modest 3 percent over the past year. That’s great news, but a debate is raging about whether this is caused by a weak economy (and therefore will reverse as the economy recovers) or other factors (and therefore may persist, drastically improving the budget outlook).