This project contains additional material for the manuscript "Proximity and Similarity Shape Friendships: Evidence from a Large Randomized Classroom Experiment". [Supplementary Material][1], including an explanation for all deviations between the pre-analysis plan and the analyses reported in the manuscript The [Pre-Analysis Plan][2], which can also be found [on the RCT Registry of the American Economic Association][3]. A summary of the [results from alternative model specifications][4]. A component including [all analyses scripts][5], as well as more detailed [outputs from all models reported in the manuscript][6] and the corresponding outputs for alternative model specifications ([models controlling for classroom size][7], [linear probability models][8]). A subcomponent including [additional analyses concerning attrition][9], including a [more detailed description][10]. And, last but not least, a component including [the de-identified data][11] underlying our central analyses (publicly made available upon manuscript acceptance). [1]: https://osf.io/anhsk/ [2]: https://osf.io/r42mc/ [3]: https://doi.org/10.1257/rct.2895-1.0 [4]: https://osf.io/kfn83/ [5]: https://osf.io/sm7up/ [6]: https://osf.io/qsaj4/ [7]: https://osf.io/avzbx/ [8]: https://osf.io/8aw3x/ [9]: https://osf.io/dxm6s/ [10]: https://osf.io/d8hpx/ [11]: https://osf.io/wf3nm/