Thank you, Alfonso! I see how having one mean would make sense when all of the runs were collected at the same time and not expected to be different. I had an experience similar to that of Shayla's. For an intervention design, I think both of your ideas would help a lot. (The only thought I had about where to offer the option was what if in the rare occasion that the same T1 had to be used for both sessions for some reason (e.g., corrupted or accidentally not acquired), but then one could always copy the file and have it loaded in for each subject?) 

Also, thanks picking up on that error. Thankfully, we did catch that earlier.

All the best,

Mary