Recall bias refers to the systematic difference in the ability of participant groups to accurately recall information. Observational studies that rely on self-reporting of past behaviors or events are particularly prone to this type of bias.

Recall bias threatens the internal validity and credibility of studies using self-reported data.

What is recall bias?

Recall bias is a type of research bias. It can occur whenever an attempt is made to collect data retrospectively, or after the event has already happened.

Recall bias is a common problem in research studies that rely on self-reporting, such as case-control, cross-sectional, and retrospective cohort studies. The time that elapses between the interview or survey and the phenomenon under study can influence participants’ recollections.

This can lead researchers to exaggerate the correlation between a potential risk factor and a disease.

What causes recall bias?

Recall bias is caused by an inaccurate or incomplete recollection of events by study participants. Research shows that remarkable or infrequent events, such as buying a house, are more memorable for longer periods of time than everyday events, such as driving to work.

In general, the following conditions increase the chances of recall bias in studies using self-reported data:

• The disease or event under investigation is significant or critical, such as heart disease.
• A participant has preconceived notions about the link between their health condition and a certain risk factor. For example, they attribute their condition to electromagnetic fields produced by nearby power lines.
• A scientifically unfounded association is popularized by the media, such as claiming a link between artificial light and increased risk of breast cancer.
• The study requires participants to report socially undesirable behaviors, for example, substance misuse during pregnancy.
• The case and control groups are different in ways that may influence their ability to recall information, such as when comparing participants with chronic pain to healthy participants.

It is important to keep in mind that recall bias is not the same as forgetfulness. If the extent of forgetfulness regarding past events is equal in the case and control groups, recall bias will not occur. If one group remembers previous events or experiences more accurately than the other, then recall bias is at play.

Recall bias example

Recall bias can cause researchers to draw false conclusions regarding the causes of an event, disease, or condition.

How to prevent recall bias

If your research involves asking participants to self-report, there are steps you can take to prevent or minimize recall bias:

• Run a pilot for your survey. Conduct focus groups or similar to find out what a reasonable recall period for the event, experience, or behavior under study is. If possible, test out shorter and longer periods, checking for differences in recall.
• Reduce the time interval between the event under study and its assessment. For example, add follow-up surveys or personal journals to reduce recall periods.
• Whenever possible, use objective preexisting records, such as medical records, rather than relying on participant experiences.
• Set up an appropriate control group, with comparable characteristics to the case group—for example, those with a different disease unrelated to the disease under study.
• Use questionnaires that are carefully constructed in order to get accurate and complete information.