Design sensitivity in observational studies

If an observational study were free of unobserved bias, we would not know this from the observed data. The best we could hope to say is that only very large unobserved biases could alter the conclusions of the study. What features of the design of an observational study reduce its sensitivity to unobserved biases? How can designs for observational studies be compared quantitatively in terms of their ability to resist unobserved biases? The talk is from (30), (31) and (39), the last joint with Dylan Small. 1. Notation and Review