Cohort Study Notes

Objectives of Cohort Study

  • Students will be able to: i. Describe cohort study design and list the uses of a cohort study. ii. Differentiate the types of cohort studies. iii. Describe key features of conducting cohort studies, including:

    • Selection of the exposed and unexposed population

    • Source of information on the exposure

    • Outcomes

    • Approaches to follow-up

    • Calculating person-time

    • Data analysis
      iv. Discuss the strengths and limitations of cohort studies.

Introduction

  • A cohort study is a type of observational analytical study.

  • Unit of observation and analysis is individuals.

  • Also known as follow-up, incident, panel, longitudinal, or prospective studies.

Definition

  • A cohort study is defined as an observational investigation in which subjects are classified based on their level of exposure to a risk factor and are followed to determine health outcomes.

Characteristics of Cohorts

  • A cohort comprises a group of people sharing common characteristics or experiences over a period.

  • Example: A birth cohort includes individuals born in the same year, sharing similar life experiences.

Study Purpose

  1. To identify risk and protective factors for diseases.

  2. To determine prognostic factors for outcomes.

  3. To describe the natural history of diseases.

  4. To identify the number of new cases in a population for planning acute care services and assessing the effectiveness of preventive measures.

Types of Cohort Studies

  • Prospective Cohort Study: Participants are grouped based on current exposure and followed into the future to observe outcomes.

  • Retrospective Cohort Study: Exposure is determined from past records, and outcomes are assessed at the study's initiation, following from the past to the present.

  • Ambidirectional Cohort Study: Combines both retrospective and prospective elements, following past records and continuing observations into the future.

Cohort Population Types

  • Dynamic or Open Population: Subjects can enter or leave at any time; losses may occur, and person-time data is used for incidence density rate calculations.

  • Fixed Population: Exposure is irrevocable; losses may occur; incidence rates are used.

  • Closed/Classical Population: Defined by an irrevocable event with set starting and ending points; no subjects would be lost.

Study Design

  1. At baseline:

    • Subjects are disease-free and classified based on exposure.

  2. Subjects are followed to document incident diseases.

Cohort Assembly

  • For infectious diseases, determine susceptible and immune individuals prior to study commencement, possibly using diagnostic tests.

Selection of Subjects

  • Ensure exposed and unexposed are free of disease and from the same population, both being accessible for follow-up.

  • All information on exposure and disease status should be available.

Data Collection

  • Characteristics of the cohort should be documented.

  • Specify measurement parameters and source, duration, and regularity of data collected regarding exposure and outcomes.

Data Analysis

  • Understand how to assess disease incidence based on exposure status:

    • Total counts:

      • Exposed with the disease: a

      • Exposed without the disease: b

      • Unexposed with the disease: c

      • Unexposed without the disease: d

Key Formulas

  1. Incidence Rate among Exposed ():
    Ie=aa+bI_e = \frac{a}{a+b}

  2. Incidence Rate among Unexposed ():
    Io=cc+dI_o = \frac{c}{c+d}

Risk Assessment

  • Relative Risk (RR):

    • Measures the association between exposure and disease likelihood.

  • Key interpretations:

    • RR = 1: equal risk

    • RR > 1: risk increases with exposure

    • RR < 1: risk decreases with exposure

Attributable Risk (AR)

  • Defined as the difference in incidence between exposed and unexposed groups:
    AR=I<em>e−I</em>oAR = I<em>e - I</em>o

Advantages of Cohort Studies

  • Offers a complete description of experiences following exposure.

  • Allows for multiple effects to be studied and clear temporal relationships.

  • Provides strong evidence for cause-effect relationships.

Disadvantages of Cohort Studies

  • Requires large subject numbers, long follow-up duration, and can be expensive.

  • Challenges in maintaining follow-up and controlling extraneous variables.

Bias Types

  • Selection Bias: Non-response varied by exposure group.

  • Follow-Up Bias: Unequal losses in exposed/unexposed.

  • Information Bias: Measurement errors and misdiagnosis.

Relevant Examples

  1. British Physicians Study (1950s): Examined smoking effects on lung cancer incidence.

  2. Framingham Study: Ongoing investigation of coronary heart disease risk factors since 1948.

Comparison of Analytical Study Designs

  • Cohort Study: Population free from condition; assesses incidence rates.

  • Case-Control Study: Compared prevalence in cases with and without characteristics.

  • Cross-Sectional Study: Describes population status at a point in time.

References

  • Newcomd RD & Marshall EC (1990). Public Health & Community Optometry. 2nd ed. Boston: Butterworths.

  • Hatch SW (1998). Ophthalmic Research and Epidemiology: Evaluation and Application. Boston: Butterworth-Heinemann.

  • Vetter N & Matthews I (1999). Epidemiology and Public Health Medicine. Edinburgh: Churchill-Livingstone.

  • Farmer R & Lawrenson R (2004). Lecture Notes: Epidemiology & Public Health Medicine. 5th ed. Oxford: Blackwell.