Epidemiologic Research Designs. Ramon Jason M. Javier, M.D., D.P.A.F.P. Department of Preventive and Community Medicine College of Medicine U.E.R.M.M.M.C.I. Objectives. To define epidemiology and differentiate the two main categories of epidemiologic work.
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Ramon Jason M. Javier, M.D., D.P.A.F.P.
Department of Preventive and Community Medicine
College of Medicine
Epidemiology is the study of the distribution and determinants of disease in human population.
Epidemiology has two main categories of work:
Descriptive Epidemiology includes activities related to characterizing the distribution of diseases within a population.
Analytic Epidemiology is concerned with activities related to identifying possible causes for the occurrence of diseases. It essentially identifies determinantsof disease occurrence.
A case report is a detailed report of the symptoms, signs, diagnosis, treatment, and follow-up of an individual patient. A case report may contain the demographic profile of an individual patient, that usually describes an unusual or novel occurrence.
Most case reports focus on:
Most case reports focus on: (continued)
A case series is a medical research study very similar in structure and form as case reports, but describes a group of cases, instead of a single patient.
This study design may be retrospective or prospective, and usually requires a relatively small sample size (usually 10 or more).
This design reports the frequency of events or outcomes of a disease, but does not show temporal relationships between events and outcomes.
This study design has case selection bias and lacks statistical validity.
In a cross-sectional study, the variables are all measured at a single point in time, with no structural distinction between predictors (exposure) and outcomes.
It may involve the measurement of current exposure and/or historic exposure.
This design is valuable for providing descriptive information about prevalence --- it is well suited to the goal of describing variables and their distribution patterns.
This design may also be used for examining associations, although the choice of which variables to label as predictors and which as outcomes depends on the cause-and-effect hypotheses of the investigators.
Uses for Cross-Sectional Study:
Strengths of Cross-Sectional Study:
Weaknesses of Cross-Sectional Study:
Cohort studies involve following groups of subjects over time. There are two primary purposes:
Cohort studies establish sequence of events and can study several outcomes. This study yields incidence and relative risk.
There are two sub-types:
Relative Risk (RR):
Strengths of Cohort Study:
Weaknesses of Cohort Study:
A case-control study is a design in which individuals with an event or condition of interest (i.e. cases) are identified and then compared with regard to one or more exposures to individuals without the event or condition of interest (i.e. controls).
A case-control study cannot yield estimates of incidence or prevalence of a disease, but it can provide descriptive information on the characteristics of the cases and an estimate of the strength of association between predictor variables and the presence or absence of disease.
Estimates of strength of association are in the form of odds ratio (OR).
OR = (AD) ÷ (BC)
Interpretation of OR:
Exposure is determined in a retrospective manner; thus, one must look back in time to assess exposure status before a person developed the disease (and became a “case”).
Ideally, exposure must be measured in a blinded manner,, to minimize bias.
Types of Bias:
Types of Bias:
A. Selection Bias
C. Recall Bias
Strengths of Case-Control Study:
Weaknesses of Case-Control Study:
In clinical trials, the investigator applies a treatment (termed intervention) and observes the effect on an outcome.
Clinical trials are essentially cohort studies, except that the groups being studied differ from each other only in the presence of a characteristic or exposure to some factor that is artificially induced.
The major advantage of a trial over an observational study is the ability to demonstrate causality.
Clinical trials are considered as the gold standard for studying interventions, specifically, randomized clinical trials.
In clinical trials, biases are minimized but not totally eliminated.
In particular, randomly assigning the intervention can eliminate the influence of confounding variables.
Blinding eliminates the possibility that the observed effects of the intervention are due to other treatments or to biased ascertainment.
There are several variations on the randomized trial design that can substantially increase efficiency, under the right circumstances:
This is a strategy for balancing baseline confounding variables that requires selecting pairs of subjects who are matched on important factors like age and sex, then randomly assigning one member of each pair to each study design.
In this design, each participant serves as his own control to evaluate the effect of treatment.
Innate characteristics, such as age, sex, and genetic factors, are not merely balanced, but actually eliminated as confounding variables.
Half of the total participants are randomly assigned to start with the control and then switch to active treatment; the other half to do the opposite.
This approach permits between-group, as well as within-group, analyses.
This may control for confounding and minimize the required sample size if carry-over effects are not a problem.
To reduce carry-over effect, untreated wash-out periods may be introduced.
Measures of Effect in Clinical Trials:
Relative Risk (RR):
For positive outcome:
Relative Risk (RR):
For negative outcome:
This design is very similar to clinical trials, except that, interventions in field trials are preventive rather than therapeutic.
Measures of Effect in Field Trials:
P1: disease rate in experimental group
P2: disease rate in control group
Protective Value (PV):
Strengths of Clinical / Field Trials:
Weaknesses of Clinical / Field Trials:
Epidemiologic research designs are either observational or experimental in character.
Each research design is suited for different objectives and purposes, and each has its own strengths and weaknesses.