بسم الله الرحمن الرحيم
By assistant prof. dr. Ali Abid Saadoon /Thiqar university / Medical college/ community medicine department
By professor dr. Ali Abid Saadoon /university of Thi-Qar / college of Medicine / community medicine department
The concept of person, place and time
Three groups of variables are commonly used in descriptive epidemiology. These are:A. Characteristics describing the persons affected such as age, sex, marital status, education, occupation, habits, genetics and ethnic groups.
B. Characteristics describing the place where persons were found affected. The distribution of the disease may have:
International
National (limited to one country
Continental
Local: only part of a country or
Urban-rural pattern?
C. Characteristics describing the time in which persons were found affected. Does the distribution follows
Secular trend (over many years and decades)?
Seasonal trend (within the same year)? Recurrent pattern or the occurrence of disease after special events, e.g., raining. ?
Distribution of disease with age
It is well known that the distribution of disease is very variable with age. Actually, age is an important confounding variable and must be considered and controlled for (there are many ways to control for the effect of age) when the distribution of disease is examined in relation to other variables. The variation of disease distribution with age may be explained as follows1. Accuracy of diagnosis. Disease is less likely to be ascertained in extreme age groups. This leads to under estimation of certain causes of death in the very young and the very elderly people. In some instances, basic population data are lacking on such extreme age groups.
2. Variation in intensity and duration of exposure to risk factors.
3. Variation in immunity and susceptibility. People are not constant in their immunological response and immunological status and are not necessarily similar to each other in that respect
4. The type of epidemiological parameter used, i.e., incidence. Prevalence or
mortality. For example, in a disease with constant incidence with age andnegligible mortality and incomplete recovery, the age specific prevalence
increases with age because cases once developed tend to accumulate over time thus raising the prevalence
5. Bimodality. In some instances the distribution of disease frequency with age may have more than one peak (bimodal) as in case of the incidence of lymphomas leukaemias, testicular cancer and tuberculosis with age. This bimodality may suggest the heterogeneity of data and the possibility that we are dealing with two disease entities rather than with one disease. For example, the first peak in the incidence rate of tuberculosis in young children is definitely primary(exogenous) tuberculosis. On the other hand, the peak late in life is mainly secondary (endogenous) tuberculosis
6. Ageing or biological clock. Some times, people become very aging and lose the ability to carry out even simple tasks, yet they have no apparent disease. They probably follow a precoded biological clock, which determines the life span.
Distribution of disease with marital status
In many studies it was reported that death rates and suicidal rates are higher among non-married people (single, widowed and divorced) than they are in married people. This is true for both males and females. Such variation might be difficult to explain but two explanations are possible:
a. Marriage stabilizes life and reduces the risk of exposure to hazardous behaviour. Married people may feel more responsible not only for their lives but for the care and life of their spouses and children. They may avoid certain risky behaviours.
People who are unmarried are actually not healthy to start with and they prefer not to marry. The higher risk of death and suicide among them is perhaps related to their poor health to start with rather than marital status (being unmarried). But this is not completely true – at least not in some countries such as Germany, where it was shown, that widowed wives live considerably longer than married wives while the opposite effect was observed for males.
Interpretation of association of disease distribution with place
The following criteria are essential to demonstrate an association of disease distribution with place:1. High frequency rates of the disease are observed in all ethnic groups living in that place.
2. Similar people who inhabit other places do not show high frequency rates of the disease.
3. Healthy people entering the place become affected by the disease at a rate similar to that of the indigenous population.
4. People who leave the place and move to other places do not experience high frequency rates of the disease.
5. Species other than man may show similar pattern of the disease
When these criteria are fulfilled, it is possible to imply that:
Either the inhabitants of that particular place posses characteristics of importance to the aetiology of the disease.Or that the physical, chemical, biological or social environment of that place contain aetiological factors of the disease. More intensive research is required to identify such factors
Interpretation of disease variation with timeSecular changes
Distribution of disease with time may follow long term changes (secular changes or trends). The changes occur over years or decades. Examples are the changes in cancer, cardiovascular disease and AIDS. Such secular changes which show a clear rise in the disease frequency with time (years or even decades) as has been shown with the rise in mortality rate due to lung cancer in some European countries during the twentieth century could be explained as follows:1. The rise indicates real increase in the incidence of the disease in response to:
a. Massive exposure to disease agents.
b. Change in life style of the people and
c. Failure of adaptation to social change.
2. The rise is artificial due to
a. Improved diagnosis of disease.b. Change in classification of disease.
c. Improved recording of cases.
d. Ageing of the population/ change in population at risk