Bios 101 Lecture 2 September 27, 2011

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 Descriptive Analytic 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 Oct;16(10): Vermeiren P, Vantilborgh A, Offner F. Myeloma of the central nervous system: report of a single center case series. Acta Clin Belg 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

Analytical Studies has comparison group Grimes et al. Lancet 2002;359:57-61

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

Cross-sectional study

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

Case-control study Grimes et al. Lancet 2002;359:431-34

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 bias Limitations (of prospective type): expensive; time-consuming; inefficient for rare diseases; selection bias, loss to follow-up in many studies

Cohort Study

Variations in Cohort Study designs Grimes et al. Lancet 2002;359: Eg: Concurrent study: Cigarette smoking and Lung cancer Hammond and Horn, JAMA, 1958, 166: Eg: Nonconcurrent study: Exposure to P32 and/or Xray increases leukemia in patients with Polycytheima vera. Modan and Lilienfeld, Medicine, 1965, 44:

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.

Experimental Designs Will continue in next class