Retirement is supposed to be the time of life when you put away your cares and worries, kick back and enjoy the wealth you’ve worked so hard to accumulate over the years. Well, maybe.

In fact, retirement for many of us is going to be an exercise in calculating odds and balancing one probability against another. Should we treat ourselves to that grand tour of Europe? Or deny ourselves because we may need the money years from now to pay for a nursing home? Should we invest aggressively to increase our chances of growing our nest egg? Or play it safe and take as few chances as possible?

These are anxiety-inducing questions and, ironically, you can blame that anxiety on the long, healthy lives we’re now living. Back in the 1920s, a newborn Canadian could expect to live for less than 65 years. Today, a baby born in Canada can expect to live to 80. So while our grandparents and great-grandparents didn’t spend a lot of time thinking about retirement — and with good reason! — we now have to budget and plan for 20 years or more of not working.

A lot can go wrong over a couple of decades. And even if you set things up perfectly for a nice 20-year retirement, fate has an odd sense of humor. After years of planning, you may die young — or live long, long past what you thought would be your expiry date.

One of the most common mistakes that people make in retirement planning is basing everything on the notion that they will live to what they believe to be the average life expectancy. You should remember that the average life expectancy is just the midpoint in a huge range of possibilities.

Among other things, bear in mind that the life expectancy figure you read in the newspaper is usually expressed in terms of what a newborn child can expect. The figure assumes there will be a steady number of deaths at every age along the way — a few people will die in childhood, a few others in adolescence, and so on. Those early deaths drag down the overall figure. So if you’ve dodged disease and accidents and made it all the way to 65, your life expectancy is considerably greater than the average for a newborn would suggest. Someone who is 65 today has a better than even chance of living to 85.

Remember, too, that the average life expectancy figures are just that: averages. Some people enjoy far fewer years; some enjoy many more. The average life expectancy for seniors may be 85, but that doesn’t mean you can ignore anything past 85. About half of seniors will live beyond that point — sometimes well beyond. The 30-year retirement is not uncommon and you have to be prepared for the possibility that you’ll be blowing out the candles on your 100th birthday.

The problem, from a financial perspective, is that there are no guarantees. Moshe Milevsky, associate professor of finance at York University in Toronto, points out that a 65-year-old man who retires today faces an 8% chance of dying before he turns 70. He also faces a nearly identical 8% chance of living past 95.

Think about the practical implications of those figures. Our 65-year-old man may expect to die relatively young. He may burn through his cash and treat himself to lots of expensive indulgences — only to find that, gosh, he’s a Methuselah who has to live the last quarter century of his life trying to make ends meet on a meager budget.

On the other hand, he could play it safe and pinch pennies to ensure he will have enough to last until he’s a centenarian. But, if so, he faces a real possibility of finding himself in a  hospital bed at 68 or 69, listening to a doctor deliver a grim diagnosis, and cursing himself for not enjoying life more when he had the chance. The odds of disappointment are identical no matter which option our hypothetical 65-year-old chooses, so how does he — or you — make a choice? The following plan can help you make the most of the retirement odds.