I have a treatment group of size 30 (30 schools in California) that used a math supplemental software. In a simple analysis, I'd like to compare students' average Math growth between our treatment group and a comparable control group. There are many schools in CA that didn't use the software. I'd like the control group to include similar performing schools (their baseline scores be similar to treatment schools with a reasonable margin of error). Also, I'd like the control group sample size be 3 times of my treatment (here 90 schools). There are many many choices of 90 schools out of more than 1000 schools in CA. How would you choose your control group?
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If I understand correctly, you want to find optimal balance in covariates between your control and treatment group. If that's so I'd use a matching procedure.
R package Matching has all the functions needed for this, including some tools to assess if balance between groups has been achieved after the matching. Check this paper with details about its use and some examples.