Selection bias

Any method that gives some groups a better chance of being included than others can push a statistic consistently above or below the parameter. Convenience samples, voluntary response samples, and a sampling frame with undercoverage are the usual ways it gets in. For example, surveying shoppers at a store on a Tuesday morning overrepresents people who are not at work on weekday mornings, so an estimate of average income can come out too low. Because the tilt is systematic rather than random, a bigger sample makes the wrong answer more precise instead of more correct.