Only a few years ago, the ambitious initiative to use shared assessments to gauge learning based on the new common-core standards had enlisted 45 states and the District of Columbia. Today, the testing landscape looks much more fragmented, with only 27 of them still planning to use those tests in 2014-15, and the rest opting for other assessments or undecided, an Education Week analysis shows.

For a variety of reasons, including the length and cost of the tests and po...