Self-Selection Bias

Groups that form through voluntary participation differ systematically from the general population — conclusions drawn from them cannot be generalized without this distortion.

• Volunteers for drug trials are systematically healthier than the general population — trial results may not apply to those who are actually sick.
• Readers of a financial investment newsletter are self-selected risk-takers — their results cannot be generalized to the broader population.
• Survivorship bias is a form of self-selection: only companies that survived are in the index — the failures were removed.