Instructor Resource

Chapter 10

Copyright © Scott B. Patten, 2015.

Permission granted for classroom use with Epidemiology for Canadian Students: Principles, Methods & Critical Appraisal (Edmonton: Brush Education Inc. www.brusheducation.ca).

Chapter 10. Cross-sectional studies

Objectives

• Define cross-sectional studies.• Differentiate between the potential descriptive and

analytical goals of cross-sectional studies.• Describe the following measures of association:

prevalence differences, prevalence ratios, prevalence odds ratios, and specific types of linear equations.• Explain how to interpret measures of association

calculated from cross-sectional data.• List strengths and weaknesses of cross-sectional

studies.

Critical appraisal

• This chapter is our first examination of a specific study design.• Identification of study designs is a key step in

critical appraisal of research reports.• Critical appraisal is more than just reading a study

and intuitively trying to identify problems with it.• Critical appraisal involves asking and answering a

series of key questions.• An early step in critical appraisal should be

identification of the study design.

Strategies for classifying study design

• A common (but sometimes problematic) approach to classifying studies is based on the motivations of the investigators.• This approach asks the question: did these

investigators have descriptive or analytical goals?• The problem is that studies can have mixed goals

and motivations may not be clearly stated.• A more reliable procedure for study-design

classification is based on actual features of the study.

What is a cross-sectional study?• Cross-sectional studies are studies in which all data

are collected at a single point in time. • Prevalence studies are usually cross-sectional.

What is a cross-sectional study? (continued)• Obviously, no study can occur exactly in a single

instant. • A prevalence study may take months or even years

to conduct. • Conceptually, however, these studies take place at a

point in time: the study design neither to looks back into the past (retrospectively), nor forward into the future (prospectively).• Note that a cross-sectional study can look back in

time with its questions (e.g., it may ask adults questions about abuse when they were children), but the design is neither retrospective nor prospective.

Examples of cross-sectional studies• The Canadian Community Health Survey• general health “iterations”• specialized “iterations”: e.g., nutrition, aging

More examples

• See abstracts at: • http://www.ncbi.nlm.nih.gov/pubmed/24044468• http://www.ncbi.nlm.nih.gov/pubmed/26001869• http://www.ncbi.nlm.nih.gov/pubmed/25935425• http://www.ncbi.nlm.nih.gov/pubmed/25929819

How cross-sectional studies look at associations• For the next few slides, consider the classic 2 x 2 contingency table:

  Disease No Disease

Exposed a b

Nonexposed c d

Prevalence differences

Prevalence Differenc e=𝑎

𝑎+𝑏−

𝑐𝑐+𝑑

Key point: A prevalence difference is an example of a measure of association because it embodies a comparison between 2 simpler parameters, which are prevalence in exposed and nonexposed groups.

Prevalence ratios

Prevalence R atio=

𝑎𝑎+𝑏𝑐

𝑐+𝑑

Prevalence odds ratios

R

Specific types of linear equationsPrevalence=α+ β 𝑋𝑒

where Xe is a variable with two values: 1 = exposed, 0 = nonexposed

This equation states that in the nonexposed,prevalence is and in the exposed it is +

Can you see that is a prevalence difference?

Strengths of the cross-sectional study designCross-sectional studies:• provide valuable snapshots of disease• describe disease occurrence across multiple

variables: they can examine multiple diseases and exposures• are cost efficient: these studies are often relatively

inexpensive and practical• are invulnerable to attrition

Weaknesses of the cross-sectional study designCross-sectional studies:• lack causal inference: temporality is usually unclear• provide no determination of risk• are inefficient for rare diseases• are insensitive to time-dependent frequency

changes

End