Abstract
The WTADJX procedure incorporated into the 2012 release of SUDAAN 11® does instrumental-variable calibration weighting using a flexible nonlinear weight-adjustment function. We review the theory behind this procedure and discuss two potential uses. The first tends to reduce mean squared errors in the absence of unit nonresponse or coverage errors. The second adjusts for unit nonresponse when the variables governing the response mechanism differ from the variables used to calibrate the weights. This occurs either because the survey variables of interest cannot be roughly modeled as a linear function of the response-model variables or because the values of the response-model variables are known only for the respondents.
