Why Unstructured Interview Have Low Predictive Validities?

Why unstructured interview have low predictive validities?
The unstructured interview is the interview without any set format but in which the interviewer may have some key questions formulated in advance. Unstructured interviews allow questions based on the interviewee's responses and proceeds like a friendly, non-threatening conversation. However, because each interviewee is asked a different series of questions, this style lack the reliability and precision of a structured interview. Also called non-directive interview. (businessdictionary.com)
Researchers have identified eight factors to answer this question.
Poor intuitive ability
Interviewer often decide and judge from candidate’s reaction after they ask questions. But people are not good at using intuition to predict behavior (Aamodt, 2008). For example no one can predict couples that whether they will fail or not. It is not easy to assume an applicant after spending only 15 minutes how well she or he will get along with the varied members of an organization? (Aamodt, 2010).
Lack of job relatedness
The most unstructured interview are common questions and not related to the particular job. Information that is used to select employee must be job related if it is to have any chance of predicting future employee performance (Aamodt, 2010).
Primacy effects
Contrast effects or the first impression it can make interviewer decide a candidate within the first five minute in the interview . The interviewer might rate a applicant’s response after each question not waiting until the end of the interview to make a judgment (Aamodt, 2010).
Contrast effects
One applicant may affect the interview score given to the next applicant if interviewers have no standard to rate applicants . Thus, the interviewer might rate the applicant’s response after each question not to wait until the end of the interview because score is different with early and after interview (Aamodt, 2010).
Negative-Information Bias
Negative-Information Bias seems to occur only when interviewers aren’t aware of job requirements (Langdale& Weitz,1973). It seems to support the observation that most job applicants are afraid of being honest in interviews for fear that one negative response will cost them their job opportunities. Thus, one might increase the accuracy of information obtained in the interview by reducing social pressure and using written or computerized interviews.
Interviewer-Interviewee similarity
The interviewee will receive a higher score if he or she is similar to the interviewer such as personality, attitude , gender or race. The research shown that interviewers gave higher rating to the same race with interviewee.

Interviewee Appearance
If applicants have physically attractive they will have an advantage in interviews over less attractive applicants and applicants who dress professionally receive higher interview scores than do more poorly dressed applicants. Interviewee appearance, it seem, is a potent hiring factor (Posthuama, Morgeson, & Campion, 2002)
Nonverbal cues
Nonverbal communication is higher correlated with interview scores. Appropriate nonverbal cues such as smiling and making appropriate eye contact (Levine & Feldman , 2002). The appropriate use of such verbal cues at tone, pitch, speech rate, and pauses is also related to higher interview scores (Barrick et al., 2008)
From above eight factors and many researches shown that unstructured interview is low predictive validity due to personal discretion can be bias. Interviewer should use structured interview because it reliability and validity.

What are the benefits to evaluate employees?
Evaluating employee performance is important in almost all firms or jobs from the lowest levels of the organization to the CEO. The organization can know how good employee performance and determine how performance strengths and weaknesses. This process is to provide feedback from employee for training as well.
Evaluating performance are for discussing defects openly and set methods for solving each defects, reviewing or updating position descriptions, clarifying job expectations and setting standards of performance.
One important benefits for evaluating employee performance is to determine an employee’s salary increase because it is not fair to pay a poor performance employee the same as an excellently performance. Making promotion decisions is the part of benefit evaluating performance is to determine which the best employee will be promoted.
Another benefits for evaluating performance are to encourage communication between employee and supervisors and also have advantages of increases in employee productivity and pay satisfaction.