The ASVAB is routinely reviewed to eliminate any potential biases from questions and scoring. The exam underwent a major revision in 2002. Two years later, a renormalization of the percentile scoring system was performed to ensure that a 50 percent score represented performing better than 50 percent of all test takers. The 10-section ASVAB improves the matching of volunteers with available jobs and helps match job openings with qualified individuals.

[1] See two documents: Sims, W.H. and Truss, A.R., A Reexamination of the Normalization of Armed Services Vocational Aptitude Battery (ASVAB) Forms 6, 7, 6E, and 7E (Alexandria, VA: Center for Naval Analyses, September 1980); and Laurence, J.H. and Ramsberger, P.F., Low-Aptitude Men in the Military: Who Profits, Who Pays? (New York: Praeger, 1991).  
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You often hear people say something like, "I got a 70 on the ASVAB." What they are talking about is the ASVAB AFQT score or Armed Forces Qualification Test Score. Many people think the AFQT score is the overall ASVAB score, but that's not correct. In fact, the AFQT score is derived from only four of the nine ASVAB subtests: Paragraph Comprehension (PC), Word Knowledge (WK), Mathematics Knowlege (MK), and Arithmetic Reasoning (AR).
Registration for the ASVAB is done entirely through your local recruiter. They will not want to start the application process without being confident that you will be a qualified applicant, so they will first ask you some information about your age, fitness, and marital status, among other things. They will likely also administer a short practice exam before taking you to a testing center to either give you a realistic idea of which jobs you would qualify for or to help you decide if it would be better to study before taking it.
After a candidate has completed the ASVAB they must wait one calendar month before retaking the exam. An additional calendar month must pass before retesting a second time. Six calendar months must pass before retaking the test a third time. The scores received from the ASVAB may be used for enlistment for up to two years from the initial test date. 
One ASVAB sub-measure, the Armed Forces Qualification Test (AFQT)--which is just a separate analysis of the same tests that focuses only on four of the subtests--is also population-referenced, but its score from 1-99 is a percentile score. This means that if you score a 73, you scored as well as or higher than 73% of test-takers in the national sample.