st: replace missing values

["Cathy L. Antonakos" <cathya@umich.edu>]

I have a dataset with hospital and ICU data. I'd like to replace missing data for one ICU with the average value of the other 3 icu's at that hospital. The dataset looks like this:

Unit_ID  Total_Score
11       . 
12       90
13       60
14       27

I can get the average for total score for the 3 units by using "if" and specifying the unit id's. But how can I then replace the missing unit's score with the average from the 3 other units? I've tried several ways and searched online but can't find an answer to this specific problem. 
Thanks for any help you can provide.
Cathy
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