This paper documents the bias introduced by attrition of individuals from administrative data with an application to the labor market consequences of postsecondary education. Attrition due to crossstate migration is non-trivial, particularly for high-earners, graduates from selective universities, and certain majors. Consequently, the premium associated with graduating from a most selective university is 23% higher than in-state earnings suggests, though this magnitude differs across context. The impact of obtaining a 2-year CTE credential is also understated, as are earnings differences across majors. Differences in missingness are systematically related to bias in measurement; we evaluate approaches to quantifying that bias.