Hi Rick, Thanks so much for this information. I had the need to compute covariance for partially paired data, where two readers each read some of the same but some different cases, and I would determine the the shared cases using the intersection or location function. But I needed a way to indicate when that resulted in the empty set. So, you had just the information I needed. Really appreciate your posts about IML. Just bought your two books a couple of weeks ago. Keep up the good work -- it is greatly appreciated!