Bias and Confounding
Non-Response Bias
-poor response in postal questionnaires -bias introduced if response is related to exposure and outcome
How to deal with confounding (2)
1) Design - randomization, restriction, matching 2) Analysis - stratified, multivariate regression
To help assess the role of chance, look at (2)
1) confidence interval around measure of effect 2) p-value
How does misclassification occur? (3)
1) error in instrument 2) subjects give incorrect info 3) researcher records info incorrectly
Steps to get the study right in order to fix bias (6)
1) select an appropriate study population 2) maximize response rates 3) minimize loss to follow-up 4) use objective rather than self-reported measures of exposure/outcome 5) follow standardized procedures in data collection 6) blind interviewer
Selection Bias is an issue particularly in what kind of study?
Case-control studies
What studies are Loss to Follow-up most relevant in?
Cohort studies
Solution to Chance
adequate sample size
Stratification
analyze data for each level (strata) of the confounder
Sampling Error
any finding could be a chance finding
What makes bias unlike confounding?
cannot correct for bias
Recall Bias
cases are more highly motivated to recall use of exposure than controls
Factors on the __________________ pathway should not be considered as confounders
causal
Selection Bias
criteria for inclusion of cases or controls is related to exposure of interest
Reverse causation
did the exposure cause the disease, or the disease cause the exposure?
Loss to Follow-up
drop-out related to exposure/outcome
Differential Misclassification
error in classification of disease status that is dependent upon exposure status or vice versa -can bias estimates of association in either direction
Non-differential (random) Misclassification
error in exposure that occurs equally for diseased and non-diseased -leads to measure of effect that is closer to unity (no effect)
What causes confounding?
estimate of exposure-outcome association is distorted by some other exposure (confounder) which is associated with the outcome and with exposure of interest
Matching
in case-control studies, can match a control to each case based on the potential cofounders -chose a control of the same age and gender of the case -must use special statistics to analyze data
The confounding factor must be associated with the disease how?
independently - not relying on any other factor
Observer/Interviewer Bias
interviewer may probe cases about their past exposure more than controls
What is confounding?
see an association between exposure and disease, but *not true association*
Types of bias (2)
selection bias information bias
Solution to Bias
study design
Solution to Reverse Causation
study design
Solution to Confounding
study design and/or analysis
Restriction
study one level of the confounding variable only -may make results less generalized/relevant to actual population
Information Bias
study subjects are misclassified either according to their disease status, their exposure status, or both
What is bias?
systematic error in an estimate arising from problems in the study design/execution
Reporting bias
those with disease are more likely to report exposure than healthy controls
Healthy worker effect
workers are generally healthier than general population -if comparison group is general population, may appear that occupation "protects" against disease
