Bias are possible in many ways in a research activity. Please see some of the bias mentioned by A. Indrayan in the book, Basic Methods of Medical Research by AITBS Publishers, J-5/6, Krishan Nagar, Delhi - 110051.
Varieties of Bias:
bias in concept
bias in design
bias in selection of subjects
bias due to concomitant medication or concurrent disease
instruction bias
length bias
bias in detection of cases
lead-time bias
bias due to confounder
contamination in controls
Berkson's bias
bias in ascertainment or assessment
interviewer bias or observer bias
instrument bias
Hawthrone effect
recall bias
response bias
repeat testing bias
mid-course bias
self-improvement effect
digit preference
bias due to non-response
attrition bias
bias in handling outliers
recording bias
bias in analysis
bias due to lack or power
interpretation bias
reporting bias
bias in presentation of results
publication bias
