The IRT model underlying ASVAB scoring is the three-parameter logistic (3PL) model. The 3PL model represents the probability that an examinee at a given level of ability will respond correctly to an individual item with given characteristics. Specifically, the item characteristics represented in the 3PL model are difficulty, discrimination (i.e., how well the item discriminates among examinees of differing levels of ability), and guessing (i.e., the likelihood that a very low ability examinee would respond correctly simply by guessing).
Again, the ASVAB is a wide-ranging exam covering many different areas. The designers of ASVAB practice tests, at least the high quality ones, know this and have spent time researching past tests to come up with practice tests that feature questions covering areas commonly tested for on the real exam. Thus, taking practice tests is a great way to focus on the material that matters most and avoid wasting your time studying content that likely won’t be on the test.
The AFQT has been used in non-military settings as a proxy measure of intelligence, for example, in Herrnstein & Murray's book The Bell Curve. Because of the test's significance both inside and outside military settings, it is important to examine what the test measures, i.e. to evaluate the construct validity of the AFQT. Kaufman's 2010 review stated that David Marks (2010) scanned the literature for datasets containing test estimates for populations or groups taking both the AFQT and tests of literacy. One study on nine groups of soldiers differing in job and reading ability found a correlation of .96 between the AFQT and reading achievement (Sticht, Caylor, Kern, & Fox, 1972). Another study showed significant improvements among Black and Hispanic populations in their AFQT scores between 1980 and 1992 while Whites only showed a slight decrement (Kilburn, Hanser, & Klerman, 1998). Another study obtained reading scores for 17-year olds for those same ethnic groups and dates (Campbell et al., 2000) and found a correlation of .997 between reading scores and AFQT scores. This nearly perfect correlation was based on six pairs of data points from six independent population samples evaluated by two separate groups of investigators. According to Marks, "On the basis of the studies summarized here, there can be little doubt that the Armed Forces Qualifications Test is a measure of literacy." However, it is important to note that AFQT has been shown to correlate more highly with classic IQ tests than they do with one another, and that the "crystallized" intelligence measured by AFQT is measured very similarly by Wechsler, in particular.
Each branch of the military sets an AFQT score requirement for applicants. The lowest qualifying score is 31, which is needed to join the Army. Higher scores are required for other branches of the military. For example, the Coast Guard only considers applicants with a score of 40 or more. Prospective recruits who have a GED rather than a high school diploma will, in most cases, need to score at least 50 on the AFQT.
If you are enlisting in the military, good - a patriotic journey, where you can build skills and see the world. However there is one thing your recruiter didn't tell you. Your ability to get into a department that you want, depends on how well you can take a quiz - just like you did with the SAT, or whatever those test are called in current times. Those test asked those really stupid questions about the cost of ice cream if a train leaves New York at 4:00 and it is 20 degrees on Colorado. This book will teach you how to answer those ridiculous questions that WILL control your destiny. Get a copy and memorize it... then serving your country will lead to a great career.
Command over the U.S. Armed Forces is established in the Constitution. The sole power of command is vested in the President by Article II as Commander-in-Chief. The Constitution presumes the existence of "executive Departments" headed by "principal officers", whose appointment mechanism is provided for in the Appointments Clause. This allowance in the Constitution formed the basis for creation of the Department of Defense in 1947 by the National Security Act. The DoD is headed by the Secretary of Defense, who is a civilian and member of the Cabinet. The Defense Secretary is second in the U.S. Armed Forces chain of command, with the exception of the Coast Guard, which is under the Secretary of Homeland Security, and is just below the President and serves as the principal assistant to the President in all defense-related matters. Together, the President and the Secretary of Defense comprise the National Command Authority, which by law is the ultimate lawful source of military orders.
As of 2017, the U.S. spends about US$610 billion annually to fund its military forces and Overseas Contingency Operations. Put together, the U.S. constitutes roughly 40 percent of the world's military expenditures. The U.S. Armed Forces has significant capabilities in both defense and power projection due to its large budget, resulting in advanced and powerful technologies which enables a widespread deployment of the force around the world, including around 800 military bases outside the United States. The U.S. Air Force is the world's largest air force, the U.S. Navy is the world's largest navy by tonnage, and the U.S. Navy and the U.S. Marine Corps combined are the world's second largest air arm. In terms of size, the U.S. Coast Guard is the world's 12th largest naval force. 
You are tested in four critical areas — Arithmetic Reasoning, Word Knowledge, Paragraph Comprehension and Mathematics Knowledge (see below) — these scores count towards your Armed Forces Qualifying Test (AFQT) score. The AFQT score determines whether you’re qualified to enlist in the U.S. military. Your scores in the other areas of the ASVAB determine how qualified you are for certain military specialties. Score high, and your chances of getting the specialty/job you want increase. The better score you get, the better chance you have of pursuing the specialty or field that you desire.
The test is part of the larger ASVAB Career Exploration Program. The Program uses the test to help students identify both their interests as well as their strengths in three skills areas (verbal, math, and science and technical skills). Based on a student's skill levels, information is provided about more than 400 occupations in order to enable students and parents to judge their potential success in areas that interest them the most. Schools that may be facing budget cuts or finding themselves with limited resources devoted to career counseling are encouraged to find out whether using the ASVAB Program would be useful, as the testing and career development services are free of charge.
The Armed Forces Qualification Test (AFQT) is used by all of the Services to determine if an applicant is eligible for the military. Four of the ASVAB subtests are combined to form the AFQT. It measures general cognitive ability and is composed of verbal and mathematics subtests. AFQT scores are grouped into categories for reporting purposes. The table below shows the AFQT categories and the percentile score ranges corresponding to the categories. Applicants that score in AFQT category IIIA or higher may qualify for enlistment incentives.