International Journal of Osteoarchaeology, vo l . 6: 320-322 (1 996)

CORRESPONDENCE

Prevalence Studies in Skeletal Populations

In Counting the Dead, ' Waldron discusses prevalence studies in the determination of disease frequency in antiquity. Although he is correct to say that palaeoepidemiological studies are cross-sectional in type, there are some features of this method as applied to skeletal populations that he has overlooked, and which I consider nullify some of his conclusions.

A typical cross-sectional study in a living population will include representatives of all age groups and, if properly conducted, will be a true reflection of the population as a whole. I f the study is repeated on another occasion, although different individuals will be taken into the study, the age structure of the study base will be equivalent unless there has been a shift in the population in the period between the two studies. I f the time frame between the two sample events is relatively short, it is possible that some subjects could appear in both studies but in different age groups.

In a cross-sectional study of an archaeological assemblage, the 'subjects' can appear only at the time of their death and they will thus appear in the age group into which they are placed by the anthropologist making that decision. Because it is well known, and shown in Waldron's illustrations (figures 2.3 and 2.4), ' that there is a greater number of older subjects in a dead population than in the corresponding living population, this must mean that there will be an overrepresentation of 'elderly' subjects in the archaeological group, and that the younger age groups will be underrepresented. This process can be likened to the bunching up of people in a queue when the front of the queue is brought to a sudden halt. Two things follow from this. Firstly, prevalences will tend to be overestimated with increasing age (and underestimated at younger ages), and secondly, the prevalences calculated

CCC 1047-482%'96/030320-03 0 1996 by John Wiley & Sons, Ltd.

from a skeletal group will not be representative of those in the living population, as Waldron asserts (pp. 48-52).'

James Bird, Imperial College,

London

Reference

1. T. Waldron. Counting the Dead. The Epidemiology of Skeletal Populations. Chichester: Wiley, i 994.

Prevalence Studies in Skeletal Populations: a Reply

J. Bird is correct to say that cross-sectional studies in palaeoepidemiology differ from the conventional form in the living. H e points to the fact that the 'subjects' can appear only at the time of their death, but there are some other ways in which the conventional and palaeoepidemiological methods differ and I have illustrated these in Figure 1 .

A typical cross-sectional study in a modern population (Figure 1 ( I ) ) , let us suppose to estimate the prevalence of osteoarthritis-a disease of considerable interest to those who look at human remains-will be composed of a number of individuals of different ages and who are sampled at a specific time, let us call it tf for convenience. In the figure, a small number of the individuals in the study are represented by the horizontal lines, the length of which is used to indicate their age, both at the time of the survey and subsequently.

A proportion of the group will be found to have the disease (as shown by the presence of the black circles intersected by the line t i ) , and from this number, a knowledge of the total number in the sample and their ages at the time of the survey, the age-specific prevalence of the disease

Correspondence 32 1

1

I 1 3 oE - I

Figure 1. Diagrams to show features of a conventional cross- sechonal study (1 and 2) and a serrotic study (3). Individuals in the studies are indicated by horizontal lines. In (l), the study is carried out at tl and those with the disease of interest at the time are shown by the black circles. With increasing time, other individuals may develop the disease, as shown by hatched circles in (2). In the serrotic study, there is avariable time from death to the time of the study (t2).The age at which individuals are considered to have the disease will be the age of their death, indicated by the fact that the circles are now at the end of the line.

is simply calculated. Following the survey, the individuals who took part will survive for varying lengths of time and subsequently some will develop the disease (shown in Figure i (2) as hatched circles) at different ages. If this cross- sectional study had been conducted amongst some relatively discrete group-the population of a single town, let us say-then further studies might include some of those who took part

previously in addition to individuals included for the first time. A population has a dynamic character because individuals are constantly being lost t o it and recruited into it, but, it is to be expected that a series of cross-sectional studies carried out on it will produce similar results and that the age-specific prevalence of any disease will be found to remain more or less constant in the absence of influences acting to change it.

One of the features of a cross-sectional study carried out on a contemporary population is that, conceptually, it is has a nice sharp edge. It is a bit like slicing through a salami and holding up the clean-cut surface to count the fat globules, further slicing will reveal other clean surfaces to count, but the number of fat globules in each slice will not vary much through the length of the sausage, all things being equal.

Consider, by contrast, the state of affairs when a prevalence study is carried out on a population of skeletons (Figure 1 (3)). Firstly-and this may seem too obvious to mention-the group comprises individuals who are dead, and so the population is entirely static, none can be lost from it or recruited into it. The ages at death of the individuals will usually vary considerably and they will have been dead for variable periods by the time the study is undertaken (at time t2, say). Thus there will be no clean edge to the population front as seen in ( I ) , rather, because the time between each individual's death and t2 will be different, this front will be uneven. These characteristics are so unlike those of the normal cross-sectional study that 1 believe the historical cross-sectional study should be given a separate name in order to emphasize the differences; I propose that the historical cross-sectional study should be referred to as a 'serrotic' study, meaning cut with a ragged edge. (From the Latin serrutus (saw) and the medical suffix 'otomy' (to cut). I have followed the venerable tradition of mixing Latin and Greek derivations to produce a scientific neologism.)

The second feature by which the serrotic study differs from its contemporary counterpart is the one to which Bird refers, that is, the age at which the individuals in the group are seen to have their disease is, in each case, their age at death (as shown in Figure 1 (3)). The problem of 'queuing' which Bird mentions, however, does

322 Correspondence

Skeletal

Figure 2. Diagram to show passage of individuals from a living to a dead population. Three age classes (40-49, 50-59 and 60+) are indicated in the boxes in the top of the picture and labelled with the first age in the class. The boxes are shown three dimensionally in order to illustrate that passage from above to below takes place over a long period. As individuals die they pass into the dead (skeletal) population as indicated by the arrows.

live subjects are represented schematically in Figure 2 by three boxes which can be considered to consist of all the individuals in the age groups shown, integrated over the whole period covered by the burials. The members of the different age classes will include both those with and without a disease in which we may be interested, but unless having the disease leads to an increased likelihood of dying at a particular age, then one can assume that all members have an equal chance of dying. Those who die will drop (literally, one might suppose) into the burial population, in the proportion of diseased to non-diseased as was present in the living population of which they once were part. The dead population will, therefore, reflect the living with respect to the prevalence of many diseases and I consider that my original contention was correct.'

T o n y Waldron Palaeopatbology Study Group,

Institute of Archaeology, University College London

not arise because of the fact that in palaeoepidemiology we are dealing with a static and not a dynamic population.

Each of the age groups in the burial population 1 , Waldron, T. Counting the Dead. The has been derived from the corresponding Epidemiology of Skeletal Populations. Chichester: population of live subjects (see Figure 2). The

Reference

Wiley, 1994: 48-52.