What is selection bias?

What is selection bias?

• A. A bias introduced by loss to follow-up.
• B. Systematic differences between those selected for a study and those not selected, causing biased results.
• C. Bias due to misclassification of exposure.
• D. Random sampling error due to small sample size.

Answer: (B)

Selection bias occurs when the way participants are chosen for a study (or who stays in the study) creates systematic differences between those included and those not included, and those differences affect the results. When selection is related to both the exposure and the outcome, the observed association can be distorted and not reflect what would happen in the target population. Examples help: if a survey on health habits mainly attracts health-conscious volunteers, the results may not represent the general population, leading to biased conclusions about the relationship between a behavior and a health outcome. In this sense, the best description is that there are systematic differences between those selected for a study and those not selected, causing biased results. Loss to follow-up is a specific way selection bias can arise (attrition), but selection bias encompasses the broader issue of representativeness. Misclassification of exposure is a type of information bias, and small sample size leads to random sampling error rather than bias.

Selection bias occurs when the way participants are chosen for a study (or who stays in the study) creates systematic differences between those included and those not included, and those differences affect the results. When selection is related to both the exposure and the outcome, the observed association can be distorted and not reflect what would happen in the target population.

Examples help: if a survey on health habits mainly attracts health-conscious volunteers, the results may not represent the general population, leading to biased conclusions about the relationship between a behavior and a health outcome.

In this sense, the best description is that there are systematic differences between those selected for a study and those not selected, causing biased results. Loss to follow-up is a specific way selection bias can arise (attrition), but selection bias encompasses the broader issue of representativeness. Misclassification of exposure is a type of information bias, and small sample size leads to random sampling error rather than bias.