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Presented at a Webinar on Research Methodology in Health Sciences at Northern Area Armed Forces Hospital (NAAFH) on 5th September 2022. By Professor Omar Hasan Kasule Sr. MB ChB (MUK). MPH (Harvard), DrPH (Harvard) Professor of Epidemiology and Bioethics
OVERVIEW:
- Study design = strategy of capturing data to determine the causal relation between the independent (determinant) variable and the dependent (outcome) variable.
- There are 2 strategies of study design: non-experimental (observational) and experimental (interventional).
- The bulk of epidemiological studies is non-experimental. Non-experimental studies are much cheaper and easier to undertake than experimental studies.
OBSERVATIONAL STUDIES:
- Non-experimental (observational) studies have no intervention, they just observe and get the data. They allow nature to take its course and just record the occurrences of disease and associated risk factors.
- Descriptive observational studies describe the what, where, when, and why of a disease. They also provide information on incidence and prevalence by factors such as socio-economic status, sex, place, and time.
- Analytic observational studies are concerned with etiology. They seek to establish the causal relation between the independent and dependent variables.
EXPERIMENTAL STUDIES:
- Experimental (interventional) studies involve deliberate human action or intervention on the independent variable whose outcome is then observed.
- Experimental studies are more difficult logistically, cost more, and have ethical issues especially if humans and animals are involved.
- Laboratory experiments have fewer ethical issues but may be expensive.
STRENGTHS OF OBSERVATIONAL STUDIES:
- The main advantages of observational studies are their low cost, ease, convenience, and lack of ethical controversies.
- A cheap study is made of a wide variety of human experiences by just observing and recording information. This is much cheaper than experimental studies in which people must be subjected to various treatments and exposures at the experimenter’s cost.
- Ethical problems in observational studies are much less than those in experimental studies because the human subject is not exposed to any major physical risk.
WEAKNESSES OF OBSERVATIONAL STUDIES:
- The non-experimental design has no random assignment or controls overall conditions making it liable to confounding effects.
- Several unplanned co-factors (giving rise to confounding, interaction, or effect modification) are involved making interpretation difficult.
- It is not possible to study etiology directly in observational studies because the investigator does not manipulate the exposures. Etiology is studied only indirectly by comparing disease experience in the group exposed to a putative risk factor with the group that was not exposed.
- Information on the variable of interest may not be available recourse is being made to surrogate variables. Mortality and prevalence can be used as surrogate variables for incidence, the variable of interest. They are however not perfect representations of incidence.
STRENGTHS OF EXPERIMENTAL
(INTERVENTION) STUDIES:
- Good control for extraneous variables (confounders).
- Randomization (random allocation) 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.
WEAKNESSES OF EXPERIMENTAL STUDIES:
- 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.