First it’s necessary to explain why ‘median’ age is so critical. Of the three types of averages, the most misleading is the mean. Because it is derived by adding up all of a set of numbers and then dividing by the number of individuals, means can too easily be dragged off center. In this case, to take an extreme example, if a community consisted of mostly centenarians and small children (with a few young adults), the mean age would be somewhere around 50–but this would be very misleading in terms, say, of what medical facilities would be needed when.
The modal age would be more useful than the mean for planning purposes. As an example, for planning as to how many school desks a community would need, the modal age would be extremely useful. Unfortunately, it’s often very difficult to get information about modes. I can’t say why. For some reason, people seem only to publish information on modal age at death, which seems not very useful for anybody not in the mortuary field. The mode, by the way, is the peak of a graph: the point with the highest number of individuals. In a ‘normal’ (aka ‘bell’) curve, the mode is near the center. In a society with high infant mortality, the mode is nearer birth, since more babies are born than live to puberty. In a society with good medical care, the mode may move over time, especially if there’s a period of high birthrates, preceded and/or followed by periods of lower birthrates; most notably, in recent years, the ‘Baby Boom’. And of course, there may be more than one mode in a distribution, as birth and death rates have fluctuated over time.
So while it would be useful to know the modal age of populations (and I’d like to encourage anybody who’s considering collecting such material), it’s not easy to get this information.
Enter the median age. The median age of a population is (by definition) the most central. The median age is the middle age: half the people are younger, half older. If it’s also the modal age, a large number of people are EXACTLY the median age.
Median age is very useful for planning purposes, because it’s possible to extrapolate the needs of people. In areas with high median ages (say above 40), there will obviously be a greater need for geriatric services in the near future. In areas with low median ages, there may be a greater need for obstetricians.
Of course, it’s also important why median ages are what they are. In areas which have undergone disasters, natural or artificial, median ages may drop drastically…for a time. Then they will begin to creep up again, because disasters are often followed by an increase in birthrates, but then the newborns age, as well.
In some areas with relatively stable economies and good infrastructure, median age rises. But then young people often immigrate to those places, which means that the median age will drop somewhat, and stabilize at a younger level…and there will continue to be fluctuations over time, as people move from one area to another, raise families, and die. Thus, for example, although Florida, as a state, tends to attract older residents because of its clement climate, young people also move there, or already live there: so that Florida has one of the higher median ages in the US, but not the highest.
Median ages also vary according to other groupings (such as sex). In most places, the median age for females is higher than that for males: which has obvious impacts on things like birthrates. Selective mortality also has impacts on median ages. Thus a population with an inadequate clean water supply is likely to have a lower median age than an otherwise similar population which has got good water. More people die ‘preventable’ (also called ‘premature’) deaths, but often more people are born in such places, as well.
All of this makes it clear, I hope, why information about median ages needs to be collected and kept up to date, for planning purposes.
For today’s less easily researched question, I bring up a subject that’s come to my attention quite a bit lately. In traffic planning, there’s a tendency to ignore alleys. Granted, alleys are not a major access for vehicular traffic. But for pedestrian traffic they are important. So why don’t there seem to be any crosswalks from alleys on one side of a crossroad to another? And why, for that matter, don’t city planners seem to question attempts to ban access to alleys for pedestrians? Frankly, if more people used alleys as walkways, it would reduce traffic on the main roads: and it would increase the safety of the alleys for all users. So how about it?
treraia 