Bias occurs when a person allows a preconceived opinion or judgement to affect their view of information, people, situations, etc. Bias can be both positive or negative and intentional or unintentional.

In patient registries, bias can cause problems if participants of the registry do not represent the disease community. For example, registry participants may not include a fair representation of affected races or ethnicities or patients who have more severe disease may be over- or under-represented. Bias can be introduced if questions in the registry are written in a way that is more easily understood by one culture or requires a high reading level. Questions can also be written to unintentionally lead to a specific answer.

In clinical research, researcher bias can affect the way responses to an intervention are interpreted or recorded. Researcher bias may happen unintentionally because the researcher may have spent years developing an intervention and strongly believes the intervention will help patients. In some cases, research bias may be financially motivated. Bias can also affect the way clinical trial results are analyzed and interpreted.

Participant bias can occur when participants in a clinical trial know they are receiving a promising therapy or the placebo and overstate or understate how it is affecting their disease symptoms depending on their expectations.

Randomization and blinding are two methods used to reduce both researcher and participant bias. However, bias can still be introduced into clinical trials (like patient registries) if the participants are not representative of the whole patient community.

Sourced From
National Institute on Aging (NIA) Glossary of Clinical Research Terms
Bias in research [2013 published article]