While situational interview questions test how candidates would respond to a hypothetical scenario, behavioural interview questions ask for past examples of a scenario demonstrating a trait. Structured interviews are designed to minimise interviewer bias, versus unstructured ones. Interviewer bias reduces effectiveness in identifying the best candidates. Potential sources of bias include interviewee anxiety levels recognised by the interviewer. The ‘puzzle interview’, popularised in the 1990s by technology firms, involves ‘puzzles’ with hypothetical scenarios or abstract problems. Panel interviews have a group of interviewers interviewing a candidate, while group interviews involve multiple candidates at a time. Group interviews are often less costly than one-on-one interviews as less interviewers are required per candidate.