Political Analysis, 2025, with Cyrus Samii and Junlong Aaron Zhou . Why this problem matters Many experiments and quasi-experiments suffer from the problem of endogenously missing outcomes : researchers only observe results for people who complete a key task, such as a survey, conversation, or interview, while dropout or nonresponse itself can be affected by the treatment. This creates what Slough (2023) calls the “phantom counterfactual” issue, where some units have undef