A dichotomy for groupoid C*-algebras

We study the finite versus infinite nature of C*-algebras arising from etale groupoids. For an ample groupoid G, we relate infiniteness of the reduced C*-algebra of G to notions of paradoxicality of a K-theoretic flavor. We construct a pre-ordered abelian monoid S(G) which generalizes the type semigroup introduced by R{\o}rdam and Sierakowski for totally disconnected discrete transformation groups. This monoid reflects the finite/infinite nature of the reduced groupoid C*-algebra of G. If G is ample, minimal, and topologically principal, and S(G) is almost unperforated we obtain a dichotomy between stable finiteness and pure infiniteness for the reduced C*-algebra of G.Comment: 40 pages. Version 2: Section 9.2 updated to reflect intersection with earlier results of Suzuki; thanks to Suzuki for alerting us. Proofs of Proposition 5.2 and Lemma 9.7 simplified using the refinement property (correcting an oversight in the proof of Proposition 5.2

The Allocation of Time in Sleep: A Social Network Model with Sampled Data

We analyze peer effects in sleeping behavior using a representative sample of U.S.teenagers from the National Longitudinal Survey of Adolescent Health. The sampling design of the survey causes the conventional 2SLS estimator to be inconsistent. We extend the NLS estimator in Wang and Lee (2013a) to estimate network models with sampled observations on the dependent variable. When accounting for sampling, we find that the sleeping behavior of the friends is important to shape own sleeping behavior, besides the impact of individual, family and friend characteristics

Dynamic Aspects of Teenage Friendships and Educational Attainment

We study peer effects in education. We first develop a network model that predicts a relationship between own education and peers’ education as measured by direct links in the social network. We then test this relationship using the four waves of the AddHealth data, looking at the impact of school friends nominated in the first wave in 1994-1995 on own educational outcome reported in the fourth wave in 2007-2008. We find that there are strong and persistent peer effects in education since a standard deviation increase in peers’ education attainment translates into roughly a 10 percent increase of a standard deviation in the individual’s education attainment (roughly 3.5 more months of education). We also find that peer effects are in fact significant only for adolescents who were friends in grades 10-12 but not for those who were friends in grades 7-9. This might indicate that social norms are important in educational choice since the individual’s choice of college seems to be influenced by that of friends in the two last years of high school.Social networks; education; peer effects; identification strategy