Epidemiology for Undergraduates Rajan Marina Joseph
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1Epidemiologic Methods
  1. Give a brief history of the science and define epidemiology.
  2. Describe the epidemiological approach to health related matters.
  3. List the aims of epidemiology.
  4. List the different methods in epidemiology.
  5. Compare and contrast between clinical medicine and epidemiology.
  1. List the different methods of observations in epidemiology.
  2. Classify observations and variables measured in medicine.
  3. Mention ways of representation of observations.
  1. List the steps/procedures in a descriptive study.
  2. Make definitions for study population and for health phenomenon under study.
  3. List frequently studied characteristics in a descriptive study.
  1. Differentiate between cross sectional and longitudinal studies.
  2. Briefly describe steps in documentation and data collection.
  3. List types of time trends in epidemics and types of epidemics.
  4. Define epidemic and epidemic curve.
  5. Describe time, place and person distributions and their advantages.
  6. Describe migrant studies and list its advantages
  1. Briefly describe the rationale of different analytical methods of epidemiology.
  2. Classify the analytical epidemiologic methods.
  1. Draw the basic scheme of a case control study.2
  2. List the steps in conducting a case control study.
  3. Describe the steps in conducting a case control study.
  4. List important problems in case control study and briefly describe to steps to minimize each.
  1. Briefly describe the steps in analyzing results from a case control study.
  2. Calculate odds ratio, given findings of a case control study.
  3. List advantages and disadvantages of the case control design.
  4. List and briefly describe advanced techniques in case control analysis.
  1. Define a cohort and narrate the distinguishing features of a cohort study.
  2. List the indications for a cohort study.
  3. Describe the general considerations required for a cohort study.
  4. Name the varieties of cohort study design and give appropriate examples.
  5. Narrate the steps in a cohort study and briefly describe each.
  1. State briefly the aim of analysis in a cohort study
  2. Draw a frame work and calculate relative risk, given the findings in a cohort study.
  3. Mention the other ways of interpreting risk.
  4. Mention the problems in analysis of cohort studies.
  5. Compare odds ratio and relative risk as measures of association.
  6. Compare and contrast the case control and cohort Studies.
  1. Briefly describe experimental study.
  2. List the steps in conducting an experimental study.
  3. List the different varieties of experimental studies.
  4. Classify Randomised controlled trials.
  1. Differentiate between randomized and non-randomized trials.
  2. Name the varieties of non-randomized trials with examples.
  3. Justify conduct of non-randomized trials.
  1. List Morris's uses of epidemiology.
  2. Briefly describe how epidemiology is useful in achieving each of the uses?
  1. Enumerate components of epidemiologic triads of varieties of problems.
  2. Briefly describe the concept of salutogenesis.
  3. Narrate the differences between pathogenesis and salutogenesis.

Introduction to Epidemiology1

 
SPECIFIC LEARNING OBJECTIVES
At the end of learning this session you will be able to:
  1. Give a brief history of the science and define epidemiology.
  2. Describe the epidemiological approach to health related matters.
  3. List the aims of epidemiology.
  4. List the different methods in epidemiology.
  5. Compare and contrast between clinical medicine and epidemiology.
 
HISTORY
Epidemiology is a very old word dating back to 3rd century BC. It is derived from three words in Greek: Epi=Upon + Demos=People+Logos = study of.
Epidemiology developed into a science of separate identity during the last five decades of 20th century. The large scale health interventions and studies that followed Second World War, in the United States, marks the formative years of epidemiology. There are references about epidemiological studies conducted and about epidemiological society of London in 19th century.
Many of the epidemiologists in these formative years were physicians who were challenged by the health problems of their patients. Many statisticians also became epidemiologists. They were involved in solving the causality puzzle of ill health with the help of the probability theory and statistical principles. Today epidemiologist has a separate identity. It is not enough to be a physician or statistician or both at a time to be an epidemiologist. A theoretical understanding of the principles of epidemiologic research and experience to apply them makes one an epidemiologist.4
 
DEFINITION
There are as many numbers of definitions as there are books on epidemiology. One of the simplest definitions has been given by Gaylord Anderson (Cole 1979). “Epidemiology is the study of the occurrence of illness.”
Considering the wide range of areas in epidemiologic research and the major methodologies involved a comprehensive definition would be— Epidemiology is the study of frequency, distribution and determinants of health related events. This is a synthesis of the major ideas given by expert epidemiologists.
Frequency implies measurements and summarized information. It is essential for comparing information on different problems or magnitude of same problem among different groups. Study of frequencies makes epidemiology a quantitative science. It may be expressed as rates/ratios or proportions. They are simple arithmetic expressions.
Distribution implies the study of the patterns of the health problem in a community. For example, which age group or gender group is more affected or which geographic area is affected or spared and what is the time of the year when problems are more or less, etc? These are expressed best through descriptive statistical techniques which are detailed elsewhere.
Determinants mean causative factors. Information on both frequency and distribution throws light on possible causative factors. The analytical approach in epidemiology tests the hypothesis thus made, and arrives at definite conclusions on causative factors. Biostatistics is the tool for making these measurements.
 
AIM OF EPIDEMIOLOGY
The International Epidemiological Association (IEA) has listed three main aims for the science of epidemiology. They are:
  1. To describe the distribution and magnitude of health and disease problem in human populations.
  2. To identify etiological factors (risk factors) in the pathogenesis of disease.
  3. 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.
Thus, we can see that epidemiology is a systematic search and research for causes and solutions for health related problems.5
 
APPLICATION OF EPIDEMIOLOGY
The scientifically/systematically made conclusions form the basis of diagnosis, treatment and control programmes for various diseases. These in turn form the steps taken towards–
(1) Controlling diseases and, (2) Promoting health. Epidemiology may thus be considered the ‘science’ of medicine. Classification which is the basis of all scientific generalization is an essential feature of epidemiology. The practice of this science is the process of epidemiology or public health. It is a systematic method of learning about health and disease. It is also a standardised way of learning and expressing knowledge about health and disease. It has its own technical vocabulary with definite meanings. Hence, it may also be considered the ‘language’ of health and disease.
 
THE TOOLS OF EPIDEMIOLOGY
The tools of epidemiology are clinical medicine, immunology, microbiology, pathology, demography, and statistics. None of these is exclusive of the other. The use of each of these tools for achieving a specific objective forms the epidemiologic method.
 
THE EPIDEMIOLOGICAL APPROACH
Understanding health and disease is the basic requirement for any action on ill health. The approach of epidemiologist in understanding health and illness is simple. It has only two important dimensions. They are: (1) to ask questions, (2) to make comparisons.
 
Asking Questions
All questions begin with What, When, Where, Who, How or Why. Questions may be asked about diseases, or actions taken to combat them. They may be asked about something that has happened (in past tense) or something that is going to be done (futuristic planning) or something that is being done (in present tense). Often inferences made about past leads on to plan for actions into future. Questions about present arise from a sense of critical awareness about our actions. All types of questions are important for an epidemiologist. Yet ‘WHY’ is probably the most important from a philosophical and existential point of view.6
 
Exercise 1
Frame two or three epidemiological questions.
 
Making Comparison
Making comparison seems to be the step to make causal inferences or find out answers to ‘WHY.’ This process involves finding out similarities as well as differences in two observations. As the differences between two groups are minimized they are said to be more and more comparable. As groups become more and more comparable any existing differences in exposure becomes of causal importance in any differences in outcome among the groups. This is the principle of making comparisons in epidemiology.
Epidemiologist arrives at all the conclusions by thus making comparisons based on observations made in response to questions asked.
The answer to why is often got from comparing different groups like affected and unaffected.
Some of the techniques to ensure quality of comparability are – randomization, matching and standardization.
 
The Epidemiological Methods
For achieving the above objectives various methods are used by epidemiologists. They have evolved from mid 19th century. These methods may by broadly classified into:
  1. Observational
  2. Experimental.
  1. Observational methods as the name implies involves only making observations on what has happened to persons and communities and make inferences from these observations. It may be further classified into:
    1. Descriptive
    2. Analytical
  2. Descriptive studies observe health phenomenon in terms of time / place/person.
    1. Cross-sectional surveys are observations made on populations or communities at one point in time. They give information on the community just as a cross section of a specimen gives information at that particular level of sectioning. This information helps us to calculate measurements of prevalence and 7also to make person and place distribution charts of the event studied. These in turn help us to make hypothesis on causal factors related to the event studied.
    2. Longitudinal surveys are surveys conducted over long periods in the same population. They give information over different time points. They are useful in calculating incidence rates and drawing time trends in health and disease.
Analytical Designs are methods that are designs developed based on specific hypothesis generated from cross sectional or longitudinal studies. They are used to gather data and the data is analysed to test the specific hypothesis. They may be called retrospective studies when both the exposure and outcome has occurred before the study commences. They are prospective when both exposure and outcomes are yet to occur.
Ecological studies, case control studies and cohort studies are the different analytical study designs.
In ecological study the unit of study is populations as against persons in case control and cohort designs.
Experimental Designs are methods where suspected causal situation is induced in selected and control populations and their effects are observed to prove or disprove a hypothesis. The major modalities of experimental designs in vogue currently may be classified as follows:
  1. Randomised controlled clinical trial
  2. Field trials
  3. Community trials.
Clinical trials are sometimes conducted in non-randomised fashion and without controls also when research has to be done under ethical and administrative constraints.
Table 1.1 illustrates the appropriateness of study designs against the purpose of research.
Table 1.1   Study designs appropriate for purpose of research
Purpose of research
Study Design
Evaluation of therapy
Randomised controlled trials (RCT)
Community diagnosis
Cross sectional study
Screening for disease
Cross Sectional survey
Measurement of incidence
Longitudinal survey
Study of prognosis of disease or therapy
Cohort study
Study of etiology or causal factors
RCT/cohort/case-control/case series/ case review in descending order of reliability
8
 
Clinical Medicine and Epidemiology
Clinical Medicine is the practical application of the scientific information about health related phenomenon gathered by epidemiological methods for promotion of health and treatment of ill health.
Table 1.2 is a comparison and contrast of epidemiology and clinical medicine:
Table 1.2   Comparative properties of epidemiology and clinical medicine
Epidemiology
Clinical Medicine
The unit of study is a defined population.
Unit of study is a case.
The concern of the epidemiologist is the disease pattern in a community.
The clinician is concerned about disease in a single patient.
Interested both in the sick and the healthy.
Interested only in the sick.
Makes a community diagnosis.
Makes diagnosis in an individual patient.
The epidemiologist goes to the community.
The patient goes to the clinician.
Concept is more of a holistic nature.
Concepts are more of biomedical nature.
Concepts need to be symbolized to perceive.
Easily perceived through clinical examinations and laboratory techniques.
Epidemiologists use clinical information complete their study of the community
Clinicians need the epidemiologist's conclusions to make the right diagnosis in his patient.
Epidemiological inquiries are often triggered by clinical observations.
Quality of clinical measurements are also ensured using the same tool used by epidemiologists namely statistics.
Epidemiologists must also have good clinical knowledge.
Clinicians also require sound knowledge of epidemiology.
 
Clinical Epidemiology
This is the application of statistical and epidemiological principles for making inferences on clinical phenomenon as observed by clinicians in clinical practice. One of the purposes of teaching epidemiology or research methodology compulsorily to all prospective clinical practitioners is to enable them to practice clinical epidemiology. The practice of clinical epidemiology is a big futuristic contribution to the science of medicine.9
 
Exercise 2
  1. Define epidemiology.
  2. What is the basic tool of epidemiology?
  3. Name at least 10 measures of frequency in health related events.
  4. Which aspect of epidemiology deals with disease patterns in community? What are the various diseases and health distribution patterns?
  5. What is the major function of analytical aspect of epidemiology?
  6. What are the aims of epidemiology according to the International Association of Epidemiologists?
  7. Compare and contrast clinical medicine and epidemiology.
  8. Classify the epidemiological methods based on purpose of research.
  9. Briefly, describe clinical epidemiology.
FURTHER READING
  1. Park's Text Book of Preventive Medicine 18th edition.
  1. Raymond S. Greenberg, et al. Medical Epidemiology, Lange Medical Books/McGraw- Hill,  3rd edition.