Bias and Confounding Flashcards
Validity
Relates to the absence of systematic error in a study
a valid measure of association
a study will have the same value as the true measure in the source population, except for error due to random variation
Bias
The extent to which a measure of association from a study differs from the true measure of association in the source population
What causes bias?
Systematic error
Internal Validity
the study result is valid with respect to the population under study
External Validity
the study result is valid to a wider population beyond the study population and/or source population
What is also called generalizability?
External Validity
Study Population
the subjects in the study
Source Population
Population from which the subjects were drawn
target population
population to which we may want to generalize our results
Non-differential bias
A bias that equally affects groups
Differential bias
a bias that affects one group or more than another
What are two general sources of bias?
Selection bias
Information bias
Selection bias
error in selection of study subjects
Information bias
error in measurement
Misclassification bias
Misclassify animals
Animals with no disease are classified as diseased
Confounding
an unknown factor distorts the relationship between the exposure and outcome
Self Selection bias
studies based upon volunteers- may not be representative of the population as a whole
Diagnostic bias
Diagnostic of disease may be influenced by the vet’s knowledge of the exposure and their expectation of disease
Observer variation
Different observes interpret information differently
What are some examples of Information Bias?
Observer variation
Deficiency of tools and technical errors
Recall bias
Reporting Bias
Recall bias
Those affected by a disease have a greater sensitivity for recalling exposure
Reporting Bias
Individuals with severe disease tend to have more complete medical records
Selection Bias Examples
Diagnostic Bias
Self-selection bias
Three condition a confounder must meet
Associated with exposure
Associated with the outcome
Not in the causal pathway between the exposure and the outcome
How can you reduce confounding variables?
Match the study so the confounding factor is equally respresented
Restrict animals with the confounding factor
Randomize
Stratify
Partition the results based on the confounding factor
How do you reduce the confounding variable after the study has been completed?
Stratify
