bios 101 lecture 2 september 27, 2011. hierarchy of designs expert opinion, usual practice case...
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Hierarchy of Designs
Expert opinion, usual practice
Case series and case reports
Ecological studies/Correlational studies
Time series
Observational studies: Cross-sectional, Case-control, Cohort, Hybrid
Well conducted RCTs
Systematic reviews of RCTS
Meta-Meta analysis
DescriptiveAnalytic
Experimental
Overview
• Designs of epidemiological studies tries to explain disease occurrence patterns and are used to determine whether an association exists between an exposure and disease
• Involves exposures, outcomes and confounders
• Many ways to evaluate the association between exposure and an outcome
Descriptive Designs: Individual level
• Case Report: Describe the experience of a single patient • Case series: Describe the experience of group of patients with
similar diagnosis– observe certain clinical features of a disease, its symptoms, tratment
and outcomes
• Studying symptoms and signs • Creating case definitions • Clinical education, audit and research
Unnikrishnan AG, Rajaratnam S, John GT, Stephen C. Boy with 'rainbow' urine. Nephrol Dial Transplant. 2001 Oct;16(10):2097-9.
Vermeiren P, Vantilborgh A, Offner F. Myeloma of the central nervous system: report of a single center case series. Acta Clin Belg. 2011 May-Jun;66(3):205-8.
Descriptive Designs: Population level
• Ecological Studies: used to determine whether association exists in groups or populations or over time relating to population characteristics thought to be at risk .
• Most ecological analyses are based on population case-series. • Ecological analyses are subject to the ecological fallacy.
Time-series studies
• Also called Before–after studies (indiviuals)• Examine cumulative incidence rates over time
Cross-sectional Studies/ Prevalence studies
• Characteristics: detects point prevalence; relative conditions
• Merits: feasible; quick; economic; allows study of several diseases / exposures; useful for estimation of the population burden, health planning and priority setting of health problems
• Limitations: temporal ambiguity; not suitable for rare conditions; liable to survivor bias
Szklo & Nieto. Epidemiology: beyond the basics. Aspen Publishers, 2000
Case-control study
Szklo & Nieto. Epidemiology: beyond the basics. Aspen Publishers, 2000
Characteristics: two source populations; assumption that non-cases are representative of the source population of cases.Merits: efficient in terms of time, money and effort; suitable for rare diseases; suitable for investigating diseases with long latency period (eg. Cancer)Limitations: not suitable for rare exposures; liable to selection bias and recall bias
Cohort Study
Szklo & Nieto. Epidemiology: beyond the basics. Aspen Publishers, 2000
Characteristics: follow-up period (prospective; retrospective) Merits: no temporal ambiguity; multiple outcomes could be studied at the same time from same exposure; suitable for incidence estimation, rare exposures, reduce risk of survivor biasLimitations (of prospective type): expensive; time-consuming; inefficient for rare diseases; selection bias, loss to follow-up in many studies
Variations in Cohort Study designs
Grimes et al. Lancet 2002;359:341-45
Eg: Concurrent study: Cigarette smoking and Lung cancer Hammond and Horn, JAMA, 1958, 166: 1159-1308
Eg: Nonconcurrent study: Exposure to P32 and/or Xray increases leukemia in patients with Polycytheima vera.Modan and Lilienfeld, Medicine, 1965, 44: 305-344
Hybrid Studies• Nested Case control Study
– Case control study within a cohort study– Normally 4 to 5 controls per case
• Case cross-over study– Variant of case-control study – Used for settings where risk of outcome is increased for a short time
following exposure– Cases serve as their own controls
• Case Cohort Study– whole cohort is subject to case-identification, but only a random sample
(called the sub-cohort) receives detailed exposure assessment.
– The cases are those emerging in the population (both in and out of the sub-cohort); the controls are subjects in the sub-cohort who are not cases.