In matrix sampling, a number of test booklets are printed for a particular subject, with each booklet containing a different selection of cognitive blocks. This design enables NAEP to assess the entire subject area within a reasonable amount of testing time. Different portions from the entire pool of cognitive questions are printed in separate booklets and are administered to different but equivalent samples of students. This design minimizes the assessment time required per student while allowing complete coverage of the subject being assessed. An example of matrix sampling: 62 students are selected Assessment books are assigned at random 30 students receive a mathematics assessment book 30 students receive a reading assessment book 1 student receives a math pilot test book, for a future assessment 1 student receives a reading pilot test book In each subject, there are several different books, with material overlapping across different books (BIB spiraling)