Self-selection bias
A distortion that appears when people choose for themselves whether to join a group or a study, so the group differs from everyone else in ways that also affect the result.
In plain English
Volunteers aren't a random sample. Companies that sign up for a four-day-week trial already like the idea; customers who answer a satisfaction survey care enough to answer; investors who stay with a fund for ten years are the ones it went well for. Whatever you then measure describes the volunteers, and stretching it to everyone overstates the effect.
Uncle Gary explains
Kevin is excited that 92% of companies in a trial kept the four-day week. All of them had asked to take part. Aunt Linda points out that everyone at her slimming club loses weight, too, apart from the ones who stopped coming.
Why it matters for your money
Fund and app statistics often count only the customers who stayed. Ask for the results of everyone who started, including the ones who left after a bad year.
Where we tested it
- Is Working From Home More Productive? What Happened When They Tossed a Coin
- Does a 4-Day Work Week Increase Productivity? The Trials and Who Signed Up
Related terms
All glossary terms. Educational content, not financial advice.