The following report describes an assessment instrument called the Mathematics Performance and Cognition (MPAC) interview. The MPAC interview was designed to measure two outcomes of interest. It was designed to measure first and second graders' mathematics achievement in number, operations, and equality, and it was also designed to gather information about students' cognitive processes while they solved mathematics problems. The current report focuses on the content of the 2015 MPAC interview, interview protocol, scoring procedures, and psychometric properties for the achievement focus of the MPAC Interview. The 2015 MPAC interview was administered to 856 students in spring 2015 in 22 schools located in two public school districts in a single state in the southeastern U.S for the purpose of evaluating the impact of a teacher professional development program on student achievement, cognition, and understanding in the domain of number, operations, and algebraic thinking in mathematics. Nonetheless, we expect the interview data and protocol to be usable in other ways to make a broader contribution in various aspects of mathematics education research. [For the previous report, "Mathematics Performance and Cognition (MPAC) Interview: Measuring First- and Second-Grade Student Achievement in Number, Operations, and Equality in Spring 2014. Research Report No. 2016-01," see ED577242.]