epidemiological study and its investigations

epidemiological study and its investigations

1. The word “epidemiology” is derived from the Greek words: epi “upon”, demos “people” and logos “study”.

2. Definition Epidemiology is the study of the frequency, distribution and determinants of diseases and other health related conditions in human populations, and the application of this study to the promotion of health, and to the prevention and control of health problems.

3. Other definitions are The branch of medical science which treats of epidemics ( Parkin , 1873) The science of the mass phenomena of infectious diseases (Frost, 1972) The study of the distribution and determinants of disease frequency in man ( MacMahon , 1960)

4. This broad definition of epidemiology can be further elaborated as follows: Population. The main focus of epidemiology is on the effect of disease on the population rather than individuals. For example malaria affects many people in Ethiopia but lung cancer is rare. If an individual develops lung cancer, it is more likely that he/she will die. Even though lung cancer is more killer, epidemiology gives more emphasis to malaria since it affects many people.

5. Frequency This shows that epidemiology is mainly a quantitative science. Epidemiology is concerned with the frequency (occurrence) of diseases and other health related conditions. Frequency of diseases is measured by morbidity and mortality rates.

6. Health related conditions. Epidemiology is concerned not only with disease but also with other health related conditions because every thing around us and what we do also affects our health. Health related conditions are conditions which directly or indirectly affect or influence health. These may be injuries, births, health related behaviors like smoking, unemployment, poverty etc.

7. Distribution. Distribution refers to the geographical distribution of diseases, the distribution in time, and distribution by type of persons affected. Determinants. Determinants are factors which determine whether or not a person will get a disease, include factors that influence health: biological, chemical, physical, social etc.

8. Application of the studies to the promotion of health and to the prevention and control of health problems. This means the whole aim in studying the frequency, distribution, and determinants of disease is to identify effective disease prevention and control strategies.

9. Aims of epidemiology According to the International Epidemiological Association (IEA), epidemiology has three main aims: To describe the distribution and magnitude of health and disease problems in human populations. To identify etiological factors (risk factors) in the pathogenesis of disease; and To provide the data essential to the planning, implementation and evaluation of services for the prevention, control and treatment of disease and to the setting up of priorities among those services.

10. The ultimate aim of epidemiology is to lead to effective action: To eliminate or reduce the health problem or its consequences; and To promote the health and well-being of society as a whole.

11. Uses of Epidemiology To make a community diagnosis . Epidemiology helps to identify and describe health problems in a community (for example, the prevalenc of anaemia , or the nutrition status of children) as well as the resources of the community. To monitor continuously over a period of time the change of health in a community . (for example, the effect of a vaccination programme , health education, nutritional supplementation).

12. To practice surveillance for a specific disease in order to be able to act quickly and so cut short any outbreak (example cholera). To investigate an outbreak of a communicable disease, analyse the reasons for it, plan a feasible remedy and carry it out, and monitor the effects of the remedy on the outbreak.

13. To plan effective health services. Effective services, interventions and remedies all depend on accurate community data.

14. Epidemiological Approach The epidemiological approach to problems of health and disease is based on two major foundations: Asking questions Making comparison

15. Asking questions Epidemiology has been defined as "a means of learning or asking questions and getting answers that lead to further questions". For example, the following questions could be asked: Question related to health events Question related to health actions

16. Question related to health events What is the event? (Problem) What is the magnitude? Where did it happen? Who are affected? Why did it happen? Question related to health actions What can be done to reduce this problem and its consequences? How can it be prevented in the future? What action should be taken by the community? By the health services? By the other Sectors? Where and from whom these activities be carried out? What resources are required? How are the activities to be organized? What difficulties may arise, and how might they be overcome?

17. Answer to the above questions may provide clues to disease etiology, and help the epidemiologist to guide planning and evaluation.

18. Making Comparisons The basic approach in epidemiology is to make comparisons and draw inferences. By making comparisons, the epidemiologist tries to find out the crucial differences in the host and environmental factors between those affected and not affected. In short the epidemiologist weighs, balances and contrasts. Clues to etiology come from such comparisons.

19. One of the first considerations before making comparisons is to ensure the "comparability" between the study and control groups. In other words, both the groups should be similar so that "like can be compared with like".

20. Measurement of health ( basic measurements in epidemiology) Epidemiology is mainly a quantitative science. Measures of disease frequency are the basic tools of the epidemiological approach. Epidemiology measures the frequency of disease, disability, death or health care utilization, and summarizes this information in the form of rates and ratios.

21. The most basic measure of disease frequency is a simple count of affected individuals. Such information is useful for public health planners and administrators for proper allocation of health care resources in a particular community. However , to investigate distributions and determinants of disease, it is also necessary to know the size of the source population from which affected individuals were counted.

22. For example when we say there were 500 deaths from motor vehicle accidents in City a during 1985, it is just nothing more than counting deaths in that city during that particular year. Such a statement does not convey meaning to an epidemiologist who is interested in comparing the frequency of accidents in City A and City B.

23. One of the central concerns of epidemiology is to find and enumerate appropriate denominators in order to describe and compare groups in a meaningful and useful way. Such measures allow direct comparisons of disease frequencies in two or more groups of individuals. For this epidemiologists usually express disease magnitude as a rate, ratio or proportion.

24. Ratios, proportions, and rates Ratio A ratio quantifies the magnitude of one occurrence or condition to another. It expresses a relation in size between two different factors. The numerator is not a component of the denominator. The numerator and denominator may involve an interval of time or may be instantaneous in time. Broadly , ratio is the result of dividing one quantity by another. It is expressed in the form of x: y or x/y X k

25. Example 1: The ratio of white blood cells relative to red cells is 1:600 or 1/600, meaning that for each white cell, there are 600 red cells. Example 2: Other example include: sex-ratio, doctor-population ratio, child-woman ratio etc.

26. Proportion A proportion is a ratio which indicates the relation in magnitude of a part of the whole. It is a specific type of ratio in which the numerator is included in the denominator and the result is expressed as a percentage. Example:

27. Rate Rate is the most important epidemiological tool used for measuring diseases. Rate is a special form of proportion that includes time. It is the measure that most clearly expresses probability or risk of disease in a defined population over a specified period of time, hence, it is considered to be a basic measure of disease occurrence. Accurate count of all events of interest that occur in a defined population during a specified period is essential for the calculation of rate.

28. Example:

29. A rate comprises the following elements- numerator, denominator, time specification and multiplier. The time dimension is usually a calendar year. The rate is expressed per 1000 or some other round figure (10,000; 100000) selected according to convenience or convention to avoid fractions.

30. Measurements of morbidity Morbidity: Any departure , subjective or objective, from a state of physiological well-being.