Lecture 5B, part 1 Flashcards
What is a cross-sectional study?
Observational study that examines the relationship between dzs (or health outcomes) and RFs as they exist in a defined pop at one particular point in time across a sample pop
How do cross-sectional studies work?
Sample of persons from a pop is enrolled and their exposures and health outcomes are measured simultaneously (selected without regard to exposure of dz status)
What is measured in a cross-sectional study?
Outcome prevalence in relation to exposure prevalence without regard to duration (also known as prevalence studies)
Use of cross-sectional studies
Monitor trends over time with serial, repeated cross-sectional studies
Cross-sectional studies may be ______ or _______ in nature
Descriptive, analytic
Purpose of descriptive cross-sectional studies
Assess how frequently, widely, or severely the outcome of interest occurs throughout a specific pop
Purpose of analytical cross-sectional studies
Assess the association between the outcome and various factors using an internal comparison group (but cannot disentangle RFs for dz occurrence from RFs for survival)
Design of cross-sectional studies
Study sample is taken from the target pop
Data is gathered at one time on both exposure and outcome
Groups are:
Exposed and outcome present
Exposed and no outcome
Unexposed and outcome present
Unexposed and no outcome
Strengths of cross-sectional studies
Results can be highly generalizable, esp when based on well-selected sample of the general pop
Relatively low cost and quick to conduct
Can collect prevalence of all variables at one time, multiple outcomes can be researched at once
Most suitable for descriptive analyses, and for formulating ideas for additional research
Used routinely to document the prevalence in a community of health behaviors, health states, and health outcomes (particularly chronic conditions)
Limitations of cross-sectional studies
More likely to ID prevalent cases of long duration
Temporal sequence between exposure and dz cannot always be inferred
-Not a problem when exposure is an unalterable characteristic (genetic trait)
-Possible if timing of exposures is ascertained relative to measurement of prevalence
Formulas used in cross-sectional studies
Prevalence
Prevalence ratio- analogous to the Risk Ratio
Prevalence Odds Ratio- analogous to the OR
When is it preferable to calculate the prevalence odds ratio in cross-sectional studies?
When the period for being at risk of developing the outcome extends over a considerable time (mos to yrs)
Chronic dz studies
Studies of long-lasting RFs
When is the PR similar to the RR?
When the outcome occurs over a short period of time
The prevalence of dz is low
When is PR the preferred measure of association?
Acute dz studies
Issues with interpretation of cross-sectional studies
Prevalent cases may not be representative of all cases
Affected by antecedent-consequent bias (temporality)
How can prevalent cases be not representative of all cases in cross-sectional studies?
Persons who survive longer with a dz will have a higher probability of being counted in the numerator of a prevalence proportion
Short-term survivors will be less likely counted as a case
If exposure influences survival time, then neither the POR nor PR will provide a valid estimate of the RR (interpretation subject to survival bias)
How can cross-sectional studies be affected by antecedent-consequent bias?
Bias occurs when it cannot be determined that exposure preceded dz, since both are ascertained at the same time (unlike cohort studies or trials)
May be minimized if info can be obtained on exposures that clearly preceded the first symptoms of dz
Why sample?
Sampling allows researchers to make relatively few observations and generalize from those observations to a much wider pop
Steps in the sampling process
Define the target pop
Determine the sampling frame
Select a sampling technique
Determine the sample size
Execute the sampling process
Definition of target pop
Entire group or pop you want to make inferences about
Study pop definition
Pop you can realistically gain access to, from which the sample is selected (after any exclusions)
Sampling frame definition
Actual list or units from which the sample or some stage of the sample is selected
Unit of analysis definition
Unit about which info is collected and that provides the basis of analysis
Sampling unit definition
Element or set of elements considered for selection in some stage of sampling (not relevant in single-stage sampling, but important in multi-stage sampling)
Sampling error definition
Estimates the degree of error to be expected for a given sample design
