Selection Bias
The bias introduced by the selection of individuals, groups, or data for analysis in such a way that proper randomization is not achieved, ensuring that the sample is not representative of the population.
Examples
- Polling only people who visit a particular website to gauge public opinion
- Clinical trials that exclude certain demographics, limiting generalizability
- Studying only current employees to understand workplace satisfaction (ignoring those who quit)
Why It Happens
Convenience sampling, self-selection, and non-response can distort what the data actually represents.
How to Counteract
- Ensure random or stratified sampling
- Ask “Who is missing from this data?”
- Evaluate whether the sample matches the population you want to understand