What type of bias occurs when study subjects are not randomly selected?

Selection bias occurs when the individuals chosen for a study do not represent the larger population intended to be analyzed. This type of bias can skew the results, as the selected subjects may have different characteristics compared to those not chosen, affecting the generalizability of the study findings. For example, if a health study only includes participants from a specific demographic or location, the conclusions drawn may not apply to the wider population.

In contrast, other types of bias, like information bias, arise from errors in the data collection process or measurement tools rather than how subjects are selected. Recall bias is a specific form of information bias where participants may not accurately remember past events, which can also distort data but does not relate to the selection criteria of the study groups. Surveillance bias pertains to the differences in the observation or reporting of an outcome, typically based on knowledge of a particular condition, rather than selection issues. Thus, the essence of selection bias is rooted in the recruitment of study participants rather than the accuracy of the information or outcomes reported.