Epidemiology sounds as if it ought to mean the study of epidemics. Well, it does include that but the term is much wider. Epidemiologists study the way disease and disability are distributed in populations, and they study factors that influence the occurrence of disease and disability.

Put more crudely, they study the questions of “Who gets ill?” and “Why do they get ill?” And they do this, generally, by recording and counting people and their characteristics, not by looking down microscopes at microbes.

Epidemiology would be an easy trade if we were all exactly alike – if you could see whether getting wet feet causes colds by looking at just one person with wet feet, seeing if they start sneezing, and then assuming that everyone else would respond in exactly the same way.

But people and diseases are much more complicated than that. Causes of disease don’t make everyone who is exposed to them fall ill. They just make it more likely that any one person will fall ill. Also, many diseases have a complex set of causes and these interact in different ways in different people. So epidemiologists have to work hard to look for patterns against this complicated background.

People used to say that being good at snooker in later life was a sign of a misspent youth. Well, being good at snooker isn’t quite a disease or a disability; but let’s pretend for the moment that it is and that having a misspent youth might cause this disease. How might you check whether it does?

It wouldn’t be easy to find out just by doing a survey of people now, because there is an element of time involved. You can only misspend your youth when you are young, and any skill at snooker that this causes will, presumably, last a long time after that. Many real diseases can take a long time to become apparent after exposure to their cause - you don’t get lung cancer the day after you smoke your first cigarette.

So an epidemiological study has to involve time and it also has to involve comparison. It would be no good just finding a bunch of people who had misspent their youth and seeing how good they were at snooker. You’d need to compare their snooker skills with those of people who had had an innocent youth.

You don't get lung cancer the day after you smoke your first cigarette

Indeed, making such a comparison would be one perfectly respectable way of doing an epidemiological study. You could find some young people, record how much they misbehave, and then follow them up for years to see whether they are any good at snooker when they are older. Then you could use statistical methods to compare the snooker skills of those who misspent their youth with those who behaved themselves.

But that’s not the only way to do it. We’re not investigating whether everyone with a misspent youth is good at snooker, only if misspending your youth makes you more likely to be good at snooker.

Maybe only two per cent of misspenders are good at snooker, but maybe that’s bigger than the one per cent of innocents who are good at it. In order to find these small numbers of snooker stars, you’d have to follow up a very large group of individuals for a long time (in the jargon we call a large group like this a cohort). Such a study could be very expensive as well as time-consuming.

Instead, you could find a bunch of people who, now, are good at snooker in later life, and you could ask them how they spent their youth. There has to be a basis for comparison, so you’d need another group of older people who were not good at snooker, and you’d ask them about their early years as well. If misspending one’s youth does increase one’s chance of being good at snooker, then the snooker stars would be more likely to have had a misspent youth than the snooker duffers.

This kind of a study is called a case-control study, because it compares ‘cases’ – people who have the disease in question, with ‘controls’ – people who do not have it. A case control study often involves fewer individuals than the other sort of study (called a cohort study), and it can be done more quickly than many cohort studies because it does not involve following people up through time.

But you don’t get anything for nothing. Case-control studies have drawbacks as well. Often they rely on people’s memories of past events. Maybe snooker stars remember their youths differently from snooker duffers, rather than actually having had different youths. Also there is the awkward question of where the controls (the duffers) come from.

If they are different from the snooker stars in some way that has nothing to do with their youth, then this other difference might be the cause of the difference in snooker ability, and not misspent youth at all. For instance, it would clearly be no good to choose the controls from the House of Lords if most of the snooker stars had had humbler origins – that would not be a fair comparison.

This last snag applies, in a different way, to cohort studies too. You follow up some people with a misspent youth and some with an innocent youth and perhaps you find that more of the misspenders are snooker stars. But maybe there are other differences between the groups. Maybe the misspenders were nicer to their grannies than the innocents, and maybe it is being nice to your Granny that causes snooker skill, and not misspending your youth after all.

Epidemiologists go to great lengths to avoid such problems, as far as they can, by designing their studies carefully and by using clever statistical methods to analyse the data. But the only way to avoid these problems entirely is to maybe do an experiment where some people, chosen at random, are forced to misspend their youth, while others are forced not to.

However, it seems unlikely that a medical ethics committee would sanction such an experiment! A carefully designed epidemiological study is often the only means we have for investigating the potential causes of a disease. Epidemiology cannot tell us everything we might want to know, but it can tell us many useful things.

Epidemiological studies were used to establish the link between tobacco and lung cancer - discover how they proved the risk of smoking cigarettes.