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

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

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 biasLimitations (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;359:341-45

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.

Experimental Designs

• Will continue in next class