Planning to work longer to bolster your retirement finances?

It can make a big difference, but it is tough to pull off. Half of all retirees leave the workforce earlier than planned due to a health problem or job loss.

New research confirms that blue-collar workers with physically demanding jobs are most susceptible to early retirement, but it also shows at least one type of early-retirement risk is spread much more widely across job types than previously thought.

Researchers at the Center for Retirement Research (CRR) developed a "susceptibility index" for early retirement that ranks 400 occupations on a 1-100 scale (100 is highest risk). They cross-analyzed federal and academic databases, and focused on the abilities required to do a job and measures the likelihood that those skills will decline with age.

The key finding: Highly educated professionals can be as susceptible to early retirement risk as a steelworker or truck driver.

The occupations least at risk are those where verbal skills are key. The group includes sales representatives (index ranking: 4), judges and magistrates (5) receptionists (6) and bookkeepers (10).

If your job requires a high level of cognitive skill, you might be at greater risk, depending on the type needed. "Crystallized" cognitive ability, or knowledge, tends to accumulate with age and boosts your odds of functioning well at older ages. But "fluid" cognitive ability, which includes things like episodic memory and reaction time to new information, tends to decline with age.

On the high end of the susceptibility scale are automotive service technicians and mechanics (100), electricians (96), detectives (83) and iron and steelworkers (98).

CRR's findings underscore the substantial risks to a financial plan of relying on extra years of work.

An unplanned early retirement requires stretching savings over more years, with fewer years of saving at high income levels just before retirement. It can also substantially reduce Social Security benefits.

How to know your own susceptibility number? The research will be published sometime in the next 12 months, along with downloadable charts of the index rankings.

By the way, reporters and news analysts, I am pleased to say, scored a 37, so count on me to stick around here for a while.

Mark Miller is a Reuters columnist.