The Big Lift™ (Big Lift) is a preschool-third-grade collective impact initiative in San Mateo County, California. The initiative is a partnership of the County of San Mateo, the San Mateo County Office of Education, and the Silicon Valley Community Foundation. Launched in 2012, the initiative aims to boost children's reading proficiency by third grade through four coordinated strategies, called "pillars": (1) High-Quality Preschool; (2) Family Engagement; (3) Summer Learning; and (4) Attendance. To date, the initiative involves seven school districts in San Mateo County that began implementing Big Lift services in the 2015-2016 or 2016-2017 school year. The RAND Corporation is conducting a multiphase evaluation of the initiative, including an implementation study of the four pillars that underlie Big Lift--"The Big Lift Implementation Study: Final Report" (ED594842)--and a series of annual descriptive analyses focused on the outcomes of children who received Big Lift services. This Technical Appendix accompanies the fourth report in this series of outcome studies. [For "The Big Lift Evaluation: Research Findings Five Years In. Research Report. RR-A1411-1," see ED615879.]