While research suggests that formative assessment can be a powerful tool to support teaching and learning, efforts to jump on the formative assessment bandwagon have been more widespread than those to assure the technical quality of the assessments. This report covers initial analyses of data bearing on the quality of formative assessments in middle school mathematics. Specifically, these data address the question of whether relatively short assessments can provide reliable and useful information on middle school students' understanding of conceptual domains in pre-algebra. Items and test forms were developed and tested in four domains (rational number equivalence, properties of arithmetic, principles for solving equations, and applications of these concepts to other domains), all of which are critical to eventual mastery of algebra. We tested the items with sixth-grade students in classrooms in four districts. We then pared down the items to create eight assessment forms that were further tested alongside instructional support materials and professional development. Results of this study suggest that relatively brief formative assessments focused on key conceptual domains can provide reliable and useful information on students' levels of understanding and possible misunderstandings in the domain. Six appendices are included: (1) Final list of big ideas; (2) Reliability coefficients of the test forms used in the pilot study; (3) IRT Tables; (4) Item (Category) Characteristic Curves; (5) The item characteristic curve of every item in the five domains; and (6) The detailed results of the reliability analysis and component loadings. (Contains 12 figures and 7 tables.)