The accuracy of free assessment tests varies greatly and depends on both the source of the test and the conditions in which the test is taken. Taking a practice test directly from the makers of the test is the best way to ensure accuracy.
Official practice tests are available online for many exams, including the SAT and other standardized tests, completely free of charge. If taken in a timed environment with no outside help, the results of these tests should be a fairly accurate indication of how a student may score on the actual exam. Tests from outside sources are not necessarily bad, but the quality can vary. It is a good idea to start with official materials.
Depending on which tests a student is preparing for, Khan Academy can be an excellent resource. It is a completely free service available online that provides practice materials and other content developed in partnership with the writers of various standardized tests.
Another issue that affects the accuracy of practice assessments is test anxiety. Students sometimes find that they are more nervous and less able to focus during their actual exam than when they practice at home. For this reason, taking a practice test in an unfamiliar or slightly distracting environment may provide a more accurate prediction of a student’s test score.