Lecture 5 - Cross sectional studies (ADD EXAMPLES?)

What is a cross sectional study?

Carried out at one time point or over a short time period

• E.g. a calendar year

Used to estimate prevalence of a specified outcome in a defined population

Provides a snapshot of outcome and associated factors at one point in time

When are cross sectional studies used?

• Study purpose is to determine prevalence of a specified outcome for population/subgroups at a particular point in time 

  • Often there is no prior hypothesis 

• Sometimes used to investigate associations between risk factor and outcome of interest  

How are cross sectional studies limited?

Only carried out at one point in time and so is limited 

• Give no indication of sequence of events   

• Did the exposure occur:

  • Before disease outcome? 

  • After disease outcome? 

  • During disease outcome? 

  • Cannot determine causality  

What can cross sectional studies be useful for?

Indicate associations which MAY exist  

• Useful for generating hypothesis for further research  

What is a pesudo-longitudinal study?

Repeated cross-sectional studies provide a pseudo-longitudinal study

How are cross sectional study results ensured to represent the population?

• Sample and responders are both representative

• Minimise the number of nonresponses 

How can non-responses be minimised?

• Telephone and mail prompting

• Second and third mailing of surveys 

• Letters stressing importance of replying 

• Incentives  

How can bias be introduced during sample selection?

Person may be more likely to respond when they have a particular characteristic

• Bias is introduced if this characteristic is related to the probability of having the outcome

How could door to door interviews lead to introduction of bias?

Door to door interview looking for a particular disease 

• Response rate will be highest in elderly and unemployed  

• These groups are more likely to be at home and more likely to experience higher levels of disease  

Why is loss of follow-up not an issue in cross sectional studies?

Only sample people at one time point so there’s no way for loss of follow-up

Why is it difficult to confirm associations with cross sectional studies?

Difficult to confirm associations between outcomes and exposure of long duration using cross-sectional studies 

This is due to only looking at them at one point in time

• Did outcome occur before or after exposure? 

• Did they occur at the same time? 

• Associations can therefore not be confirmed.

What are the advantages of cross sectional studies?

• Relatively inexpensive

• Short time span

• Estimates prevalence of a specified outcome

• Many outcomes and risk factors can be studied 

• No loss of follow-up

What are the disadvantages of cross sectional studies?

• Not easy to infer causal association 

• Only presents situation at one point in time which may change 

• Prevalence-incidence bias 

  • Bias due to investigating prevalence not incidence

    • Individuals who died early from the disease or recovered quickly are excluded from the study population

  • For chronic diseases, risk factors associated with death will be under-represented

MAYBE ADD EXAMPLES??