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Hiring managers are not forcing you to undergo a personality test simply for their amusement. The purpose of these tests are to help them make the most effective hiring decisions. Employers utilize the results to help them decide whether or not you have the right temperament for the position. Try to view the test as something that can benefit you, not just the employer. You can look at it as an excellent way to determine if this is a job where you can reach your full potential. Keep in mind that psychology is not an exact science. Therefore, the results of any psychological exam are never 100% reliable. You should know that prospective employers will be using the results as one of many factors in the hiring process. Feel free to ask the HR representative how heavily the test results will be weighed when choosing the successful candidate. You may or may not get hired. But remember, just because you were not the person best suited for the position, that does not mean you "failed" the exam. The employer is looking for certain traits. If you are not the best match, that simply means that you should continue your job search.
Think like an employer. Understand the validity. Be prepared for any result.