Very interesting responses! I have been curious whether neighborhood income is an acceptable substitute for individual-level income, in cases where individual income is not available. The Diez-Roux article suggests that both are important for a contextual study. This makes sense, because individuals are influenced by both their own income and the average neighborhood income in which they live. The use of individual-level variables, group-level variables, or both would depend on the question under study and the role of individual behaviors/characteristics versus group-level factors.