At What Level Should You Randomly Assign? Posted on February 15, 2006 Robert C. Pianta (bio) talks about deciding between random assignment at the school and the district level.

One of the challenges that we faced in doing random assignment was to find the level of the school system at which random assignment needs to take place. You would think that if you really want teachers assigned into conditions, but if we're teaching in the same school, and you're assigned into one condition, and I'm assigned into another condition, there's a very high likelihood of contamination, so most people who do random assignment and are interested primarily in classroom effects actually do the assignment at the school level. We did assignment at the district level in the intervention that we're doing partly because the nature of what we were trying to change was something where there was a reasonable likelihood that there could be even contamination from school to school within districts or between districts, so we randomized at the district level.

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