The concept of "prior learning assessment" is decades old, and it has grown to include multiple types of mechanisms for measuring knowledge and skills that students have accumulated through various types of formal and less formal formats, such corporate training, work experience, and independent study. The most common types of assessments include standardized exams developed by the College Board (the College Level Examination Program exams and Advanced Placement exams), the American Council of Education's guides for recognizing credit for instructional programs offered in the military and by employers, and institutional reviews of individualized student portfolios.