A person’s choice of when to retire is a singularly important financial decision, and the timing frequently is off, according to research from Morningstar.

In a new paper, Morningstar’s head of retirement research, David Blanchett, suggests that people tend to retire earlier than they had expected, and because of this uncertainty should be saving more if they plan to retire after age 65.

(Related: 2 Steps Could Help Most Clients Reach Their Retirement Goals: Morningstar)

Citing a Gallup survey, Blanchett notes that people on average people retire about four years earlier than expected. About 48% of people retire earlier than expected, based on a report from the Employee Benefit Research Institute.

Blanchett sought to understand what factors might predict who would miss his or her retirement target by analyzing decisions made by retirees over 12 survey years from the University of Michigan’s Health and Retirement Study.

Factors including gender, marital status, education, job stress level and health problems did not predict early retirement.

(Related: What’s the Ideal Retirement Age? Here’s What Americans Think)

He found that expected and actual retirement ages aligned around age 61, with those expecting to retire after 61 tending to do so before then, and those expecting to retire before the age of 61 actually retiring later.

The biggest gap was for those expecting to retire after age 61, where the difference was about a half a year for each year past age 61. For example, someone who planned to retire at 69 was likelier to do so at 65.

The effect of an earlier retirement could be significant, Blanchett found.

Consider someone who thinks he or she is on track for a successful retirement — which the study defined as reaching one’s retirement goals with a more than 90% probability of success. In fact, because of retirement-age uncertainty, this person has only about a 65% probability of success.

The study showed that a person’s retirement age was unpredictable not only because many people retire earlier than expected, but also because of the near impossibility of predicting who will be part of this group — job changes and health issues can play havoc with the best-laid plans.

This suggests that overall, people should save more for retirement. If they are looking at retirement past age 65, they should save a lot more because the odds are they will not end up working that long.

According to Blanchett, advisors and clients can model the potential uncertainty in a financial plan and understand the implications associated with what happens if the individual retires early.

Retiring early can reduce saving and investing time, lower the potential Social Security benefit and potentially increase the time spent in drawdown, making it harder to reach retirement success.

In a blog, Blanchett writes: “It’s common to treat returns as random in a financial plan; it’s time to start thinking about retirement age in a similar light.”

A behavioral scientist recently posited a couple of ways advisors can help their clients reach their retirement goals.

— Check out How to Manage Sequence of Returns Risk in Retirement: The Advisor and the Quant on ThinkAdvisor.