Selection Bias
A false conclusion that comes from comparing groups who were different from the start.
Why it matters
People who choose to do something are often more motivated, so they may have done well anyway.
How to use it
- Ask who chose to be in each group.
- Assign people to groups at random when you can.
- Treat before-and-after comparisons of self-selected groups as clues only.
Example
People who finish a survey stay longer, but they may simply be more motivated to begin with.