Nice text! Note, that when you calculate the variation in the mean, you should ONLY use the original values.
Imputation has been studied a lot in Mathemtical Statistics. My knowledge tells me that I do not know the subejct.

Another question is: Why are some values missing ? Systematic effect ?
/ Br Anders

p.s. Jens Malmros has studied this. His thesis about this subject did win him a scientific price.
He now works at SCB Statistics Sweden.