The question has been raised as to what is the difference between a test which predicts as compared to an achievement test. The ACT and SAT are two well known tests possessing predictive validity. Generally, a high school junior or senior take one of these two tests. They attempt to indicate how successful he/she will be in college course work. But, there is a standard error of measurement (SEM) inherent in the test, meaning it does not measure perfectly. Thus, for example, if a student receives a score of 18 and the SEM is 2, then the true score might vary from 16 to 20. The SAT and ACT are designed to measure predictive validity and not achievement in general.
An achievement test such as the Iowa Test of Basic Skills attempts to answer the question, "How well is the student doing presently in skills in selected subject matter areas." Achievement tests, like all standardized tests have a standard error of measurement. The kind of test used is determined by its purpose. How well will the high school student do in college? The test must then have predictive validity. Or, how well is the student doing presently in school learning, then the validity differs in that the test attempts to measure the present state of achievement. For instance, the California Test of Personality is not valid for predicting how well a student will do in college, nor will it measure present academic achievement, but it attempts to ascertain personal and social adjustment of the learner.
— Marlow Ediger, North Newton