STATISTICS AND THE LIFE EXPECTANCY OF ANCIENT PEOPLES

Statistics. It's math. It's detail work. It's hard stuff for most people to conceptualize.

Which is where the issues with Life Expectancy statistics start to come in. The trouble occurs because, when studying ancient peoples – say, Rome, with a Life Expectancy of about 30 – so many people assume that that means Roman people lived to be about thirty, and then died.

However, Life Expectancy and Life Span are NOT the same thing.

Life Expectancy is a measure of the average Life Span of a particular people at a given point in time. Life Span is a measure of how many years those people in that place at that time might expect to live.

Still don't see a difference in those two things? Well, let's illustrate.

Say there is a group of ten people on an island, and all of these ten people live to be 100. The Life Expectancy and Life Span of these people is then the same – 100 years. Why? Because Life Expectancy being an average, you add up the ages of your group and divide by the number of people (100x10/10). Life Span, on the other hand, is instead an estimate of how long a person will live based on the experiences of their generation and previous generations (which, in this case, is 100). Thus, in both cases, 100.

But, what if instead one of those 10 were to only live to 90 years? Then Life Expectancy on the island would have dropped to 99 ((100x9+90)/10). But Life Span would still be 100, because 100 is still what by far most people on that island live to be, and thus 100 is still the age that people would expect to live to.

But what if, instead, one of those 10 happened to die in infancy? Life Expectancy just dropped to 90 ((100x9+0)/10). But Life Span is STILL 100. Most people still live to be 100, even though Life Expectancy has dropped by 10 years!

Those numbers are important for several reasons. Firstly, examine how much one infant death affected Life Expectancy. In fact, nine of our ten people could live only to 90, and it wouldn't change Life Expectancy as much as does having one person die in infancy.

This is one of the tricky things about statistics, particularly averaging statistics. Large discrepancies carry much greater weight upon the total than small deviations do.

Now, let's take that lesson back to the Ancient Roman times. Romans did NOT expect to die at about age 30. However, Ancient Rome had a terrible infant mortality rate, which has the effect of greatly dragging down their Life Expectancy average. But if a Roman managed to survive past childhood, they could expect a fairly long Life Span. Indeed, Romans didn't even reach their majority (full adulthood under their law) until aged 30.

So, what was the Life Span of Ancient Rome? It greatly depended on the social status of the Roman in question (that is still a large factor in Life Span today, but the discrepancy was MUCH greater in Roman times). A poor Roman who survived childhood could generally expect to live into their early fifties, in the same way that we expect to live to be 70 today.

A wealthy Roman, on the other hand, had a much longer Life Span, and many a Roman emperor lived well past their 70^th year, and there are reports of Romans living into their 80s, 90s, and even (though rarely, and far less well documented) into their centuries. Which makes sense. Modern medicine hasn't increased humanity's Life Span that much; it has instead increased the likelihood of people living a long Life Span.

In other words, there is a lot less infant mortality these days, and modern medicine (but mostly modern health practices) have made it more likely besides that a person will live a long life.

And why is this important?

Well, for accuracy's sake – no more 'historical' talk of Romans (et al) dying at 30.

But also, this is a perfect illustration of how statistics can be misleading. (Or, more importantly, how they can even be MADE to be misleading.) Statistics without context are meaningless. Statistics without proper context are cons.