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Lecture at King Fahad Medical City prepared by Professor Omar Hasan Kasule Sr. MB ChB (MUK). MPH (Harvard), DrPH (Harvard) Professor of Epidemiology and Bioethics
- The main purposes of epidemiological research can be listed as exploration, description, explanation, and prediction.
- Exploratory studies are
preliminary and have the objective of obtaining basic information about a
disease and its potential causes in order to enable the formulation of causal
hypotheses that can be tested in more sophisticated studies.
- Descriptive studies
characterize disease in terms of place, time, and person.
- Explanatory studies seek to
establish causal relations between a disease and its risk factors.
- The prediction uses existing epidemiological profiles of a disease and its exposures to predict future disease patterns.
- A decision has to be made that a public health or medical problem exists.
- Description of the extent and
distribution of the problem.
- Hypotheses are formulated
about the causes of the problem.
- Appropriate studies are designed to test the hypotheses.
- An epidemiologic study involves data collection, data analysis, and data interpretation. Biostatistics is the technology of the scientific method that enables sophisticated data analysis and interpretation.
- Epidemiology, as a scientific discipline, uses the procedures of the scientific method.
- The scientific method requires stating a hypothesis, collecting and analyzing data to test the hypothesis, and reaching conclusions about the hypothesis.
- Continuous improvement of knowledge is by generating and testing hypotheses.
- The hypothesis is based on previous knowledge or data. It may also be based on purely theoretical or intuitive considerations.
- An epidemiological hypothesis is formulated to relate two phenomena: the disease and the putative cause of the disease (the exposure or risk factor).
- Four methods are generally used in such a formulation: difference, agreement, concomitant variation, and analogy.
- A causal hypothesis can be generated by looking for the difference between two situations, one that leads to disease and the other that does not. The difference(s) between the two situations may be the putative cause of the disease.
- The agreement involves similarities between different situations that lead to the same disease. The common factor on which the similarity or agreement is based could be the putative cause of disease.
- If variation in a putative causal factor is always associated with concomitant variation in disease occurrence, then that factor is likely to be etiologically important.
- The method of analogy is used to generate a causal hypothesis by looking at a causal relation between two phenomena and understanding its mechanism. If that mechanism is relevant to another disease situation, then the cause in the first instance could also be the cause in the second instance.
- Hypotheses must be specific and testable.
- Empirical data used to test hypotheses is from experimentation and observation.
- The conclusions from testing a hypothesis can be rejection/non-rejection and never acceptance.
- A new hypothesis is generated from the conclusion and the process is repeated.
- The use of the scientific method implies among other things that epidemiological knowledge is never stable. It keeps on changing and getting nearer the truth as new information is discovered.
- Epidemiological data can be sourced from observational or experimental studies.
- The bulk of epidemiological studies are non-experimental.
- Non-experimental studies are much cheaper and easier to undertake than experimental studies.
- A major attraction of the epidemiological non-experimental study is that it is generally a cheap source of data on human disease and appropriate risk factors.
- Experimental studies involve deliberate human action or intervention whose outcome is then observed. Their objective is to establish a definitive causal relation.
- Examples of experimental studies involving humans: are clinical therapeutic trials, community intervention studies to assess vaccines, community interventions to assess preventive measures, and animal bio-assays.
- The main characteristics of an experimental design are random selection, random assignment, manipulation of independent variables by the experimenter, and control of all other variables by the experimenter.
EXPERIMENTAL
STUDIES: natural and true experiments
- 2 Types of
experimental studies: Experimental studies are of
two types: natural experiments and true experiments.
- Natural Experiments can be analyzed to provide insight at little cost to
humans. They are experimenting that humans do not design but can observe.
- Classical examples of natural experiments are Snow's study of cholera in London, the study of air pollution episodes leading to mortality in London and Los Angeles, and the study of atomic bomb survivors in Hiroshima and Nagasaki.
- True experiments are more involved and expensive than observational studies or natural experiments but are still cheaper and easier than laboratory-based research.
- Classical examples of true experiments are Lind's trial of fruit juice for scurvy in 1747, the discovery of the relation between rice and beri-beri by Fletcher in Kuala Lumpur in 1905, and tests of the prevention of dental caries by fluoridation carried out by the United States Public Health Service (USPHS) in 1970.
EXPERIMENTAL
STUDIES: study population, study groups, and end-points
- Study population: A study population suitable for
the problem being studied must be selected. It may represent the general
population or may be restricted to certain groups.
- Study groups: The researcher determines the
experimental or intervention group (who will get the exposure studied) and the
comparison group.
- End-points: The end-points of the study must
be defined precisely.
EXPERIMENTAL
STUDIES: randomization
- Randomization is the basic sampling scheme for experimental studies and clinical trials.
- The objective of randomization is to prevent investigator bias in the selection and allocation of subjects to the treatment and reference groups, ensure comparability of results from the two groups, and increase confidence in the results of the study.
- Randomization has the objective of balancing the distribution of confounding factors, known and unknown, across groups. This purpose is not always achieved to 100% perfection.
- To obtain more balancing of confounding randomized block design is used in which the randomization process is carried out separately for each 'block'. The block could be a gender or age group.
- Once the 2 groups are identified, the study may be carried out in parallel or it may use the cross-over design.
EXPERIMENTAL
STUDIES: strengths
- The main strength of experimental studies is a good control for confounding.
- Randomization in experimental studies is the basis for unequivocal evidence of causality.
- The experimental design enables the investigator to control extraneous variables.
- The experimental design enables the investigator to vary the levels of independent variables in order to make a more thorough and detailed study.
EXPERIMENTAL
STUDIES: weaknesses
- The main weakness of experimental studies is that well-controlled experiments on humans are difficult to carry out and have ethical problems.
- It is difficult to put humans under full experimental conditions where they can be observed for 24 hours.
- Ethical controversies and violations of human rights always arise in such studies.
- Observational studies allow nature to take its course and just record the occurrences of disease and describe the what, where, when, and why of a disease.
- Observational studies precede and prepare for definitive experimental studies. They may be descriptive or analytic.
- Descriptive observational studies may provide information on incidence and prevalence by factors such as socio-economic status, sex, and place.
- Analytic observational studies are concerned with etiology. Two types of comparison are employed: disease in exposed vs. disease in non-exposed & exposure in diseased vs. exposure in non-diseased.
- The increasing deaths from lung cancer in the first half of the 20th century and cigarette smoking;
- Oral contraceptives and venous thrombosis;
- Congenital cataract and intra-uterine rubella infection;
- Low rates of dental decay in populations with mottled teeth living in municipalities with a high fluoride water content;
- Burkitt’s Lymphoma in low altitude and high rainfall areas that have a high malarial infection
- Increasing demand for pentamidine to treat P. carinii pneumonia and the HIV/AIDS epidemic.
NON-EXPERIMENTAL OR OBSERVATIONAL STUDIES: study designs
- Cross-sectional studies, also known as prevalence studies, require new data collection at a point in time.
- Case-control studies compare the distribution of risk factors in diseased individuals (cases) and disease-free individuals (controls). In a case-control study, the disease is fixed at the start and the diseased are studied to find the exposure.
- Follow-up studies also called cohort or prospective studies enable the study of temporal relations that may be prospective or retrospective. In follow-up studies the exposure is fixed at the start of the study and exposure precedes disease.
NON-EXPERIMENTAL OR OBSERVATIONAL STUDIES: strengths
- Non-experimental or observational studies are easy, convenient, and cheap.
- They enable us to understand the causes of disease by formulating and testing hypotheses,
- They explain local disease patterns, describe the natural history of the disease (death, survival or cure, and complications), and for administrative uses (health resources needed).
- The main advantages of observational studies are their low cost and lack of ethical controversies. A cheap study is made of a wide variety of human experiences by just observing and recording information.
NON-EXPERIMENTAL OR OBSERVATIONAL STUDIES: Weaknesses
- The non-experimental design has no random assignment or controls overall conditions making it liable to confounding effects.
- It is not possible to study etiology directly in observational studies because the investigator does not manipulate the exposures.
- The epidemiological methodology employs the scientific method.
- It is empirical, inductive, and refutative.
- Epidemiology relies on and respects only empirical findings.
- There is no room for preventive action being based on pure reasoning and rationalism, not subjected to empirical verification by collection and analysis of data.
- Empiricism refers to reliance on physical proof
- Induction is a type of reasoning that starts from one observation and generalizes to an explanatory theory.
- Induction is the converse of deductive inference in which we start with a theory and then predict the observations.
- Progress of science requires the use of both inductive and deductive inference.
- Both inductive and deductive logic are used in epidemiological reasoning.
- The inductive is used more because it is more in line with empirical experimental verification.
- The concept of refutation was developed by Karl Popper who reasoned that science grows by conjecture followed by refutation or falsification.
- Refutation is a type of scientific reasoning introduced by the philosopher Karl Popper that emphasizes the rejection of suppositions rather than accepting them. This is in line with the spirit of empirical scientific inquiry which never closes the door to further ideas by accepting any idea as conclusive.
- Rejecting an idea opens the way to test other ideas.
- Based on the scientific method, epidemiology can refute a finding but can never offer conclusive proof. This is in line with the spirit of empirical inquiry that knowledge and understanding grow continuously and facts accepted and interpreted in one way today may be rejected and interpreted in another way tomorrow.
- Karl Popper argued that a theory is scientific if it is falsifiable. Any assertion that is not falsifiable is not considered scientific because it cannot be subjected to scientific experimentation.
- Bayesianism was originated by the Reverend Thomas Bayes. It provides a way of changing prior probabilities into posterior probabilities.
- In other words, it enables a person to start with a prior belief that is modified by empirical observations.
- In scientific inference, probabilistic models are used more than deterministic models.
- There is an underlying deterministic order to physical phenomena that humans do not know and cannot measure. Probabilistic models are therefore used as a necessity.
- Thus, science is probabilistic. The better the research design the higher the probability of the truth.
- Limitations of human intellect and observations make human knowledge relative and never absolute.
- This leaves the door open for further scientific investigations since none of the scientific facts is absolute.
- Many can be disappointed that epidemiology is not as deterministic as laboratory science yet it has many strengths that compensate for this lack of precision.
- In a 1979 seminal discussion of the strengths and weaknesses of the epidemiological methodology, Brian McMahon, Professor of epidemiology at Harvard, summarized the strengths as obtaining observational data cheaply and with a wide range of human exposures.
- He identified the major weakness as the inability to offer conclusive proof of the disease-exposure relationship. In this sense, epidemiological findings open the way for further work and verification by laboratory science.
- One of the strengths of the epidemiologic methodology is the possibility of successful intervention against disease based on a preliminary and not full understanding of the causal pathway.
- Dr James Lind discovered that lemon could prevent scurvy in 1753 but it was not until 1928 that A. Szent-Gyorgi discovered ascorbic acid deficiency as the cause of the disease.
- In 1755 CE Gaga discovered the prevention of pellagra but its etiology due to niacin deficiency was discovered by J Goldberger et al in 1924.
- In 1755 Percival Pott discovered the prevention of scrotal cancer by avoiding soot from chimneys; in 1933 JW Cook et al discovered Benzo (alpha) pyrene as the carcinogen.
- In 1798 Dr Edward Jenner discovered that vaccination could prevent smallpox but it was not until 1958 that F Fenner discovered the orthopoxvirus as the cause of smallpox.