Selection bias

Selection bias is when the way people are put into groups - for example, if someone is picking who gets to be in a special club - is not fair. For example, someone might only invite the people they like, or who are similar to them, and not invite those who are different. This could mean the group ends up not being very balanced or diverse. It's like shuffling a deck of cards, but then not putting all the cards back in the deck before you start dealing them out - some of the cards have been left out and it affects the outcome.