What is selection bias?

• A. Systematic error from non-random sampling leading to unrepresentative results.
• B. Random variation due to small samples.
• C. Error from measurement instruments.
• D. Bias due to data entry.

Answer: (A)

Selection bias happens when the way participants are chosen makes the study sample unrepresentative of the population. It’s a systematic distortion that comes from non-random sampling or recruitment methods, so certain groups are favored or ignored and the results can’t be generalized.

For example, surveying people who pass through a specific clinic or who volunteer for a study from a single platform can overrepresent some groups and underrepresent others. That non-random selection shifts the results away from what would be observed in the true population.

This differs from random variation due to small samples, which is a chance fluctuation rather than a consistent bias; it also isn’t about errors in measurement instruments or data entry, which are different sources of error altogether.