cross-sectional study

Question 1

Definition of a cross-sectional study

Answer 1

• Observational study –> exposure of characteristic at time of the study
• Simultaneous assessment of exposure and outcome at the time of the study
• Individual is unit of analysis
• The association between prevalent disease and prevalent exposure is examined.
• limited ability to ascertain time sequence of exposure and outcome
• Tends to be used to measure factors associated with diseases with slow onset and long duration
• Not subject to ecologic fallacy – looks at people on an individual level

Question 2

Selection of target population in a cross-sectional study

Answer 2

• Identify target population
  
  • Prevalence rates of disease in a particular geographic area, age group, racial/ethnic group or occupational group
  • Sometimes based on exposure of interest, if readily identifiable
• If relatively small numbers are involved, entire population may be included or a representative sample
  
  • A community or a random sample of households in a community
  • May be a stratified random sample (e.g., based on socioeconomic status, age, etc.)

Question 3

Pros of cross-sectional study

Answer 3

• cheap and simple
• ethically safe.
• Quick
• Uses representative sample of the population
• Study sample usually representative of general population (generalizable)

Question 4

Cons of cross-sectional study

Answer 4

• establishes association at most, not causality;
• confounders may be unequally distributed;
• group sizes may be unequal.
• Since it identifies prevalence and not incidence, this may not be representative of all the people with the disease
• Potential misclassification if disease is being actively treated or if disease has exacerbations and remissions or is episodic (e.g., asthma, multiple sclerosis, lupus)

Question 5

Biases in cross-sectional study

Answer 5

Biases

• Recall bias
• Excludes those who died prior to the study (survivor bias)
• Temporal bias – (most important limitation) cannot determine if the exposure preceded the outcome
  
  • Establishes associations, but not etiologic relationship
  • Duration and timing of exposures are important to document, if possible, to try to relate onset of disease
• Potential bias in classifying both disease prevalence and exposure status (chicken vs egg)
• Biased availability for the study - Sick patients may not be easily observed

Question 6

Measure of association in cross-sectional study

Answer 6

• Measure of association is the odds ratio = (a/c) / (b/d)
• Prevalence