Glossary
Selection bias.
Direct answer
Selection bias is the distortion that arises when the cases studied differ systematically from those the conclusion is meant to cover. It appears during recruitment, during allocation to groups, or through unequal dropout during the study, and it pushes the result in one particular direction.
Selection bias cannot be cured by adding cases: a large but skewed sample merely gives a more precise wrong value. What is always revealing is who does not appear in the study, such as refused, withdrawn or untraceable cases. That absence is passed over in silence far more often than it is explained.
In equine research much data comes from university clinics and from high-value sport horses, a population monitored more closely and managed differently from the average leisure horse. In human medicine the same imbalance arises through volunteers, who are on average healthier and more motivated than the general population.
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