  Covariate benchmarking is an important part of sensitivity analysis about
omitted variable bias and can be used to bound the strength of the unobserved
confounder using information and judgments about observed covariates. It is
common to carry out formal covariate benchmarking after residualizing the
unobserved confounder on the set of observed covariates. In this paper, I
explain the rationale and details of this procedure. I clarify some important
details of the process of formal covariate benchmarking and highlight some of
the difficulties of interpretation that researchers face in reasoning about the
residualized part of unobserved confounders. I explain all the points with
several empirical examples.
