The data only exists for 1911-1926 - a much longer period which included WWI.
Well, I have seen numbers for 1919-23 (or 1918-1923 can't remember) ...
Please cite.
Can't really, it is a long time ago (maybe 5-6 years) since I read about the subject. I remember content, but rarely the exact source of info.
What about potential non-census sources? Do they exist?
Unfortunately for a primary source perspective I don't actually know, church records would not be the most reliable here (they don't keep records of parishoners but only of sacraments kept, burials, etc). All my information comes from (the massive) secondary literature on the subject. A possible major source would be land sales and deeds - Protestant depopulation after all was mainly a rural thing in the period in question - but I wouldn't know the exact details here. Sorry, this really isn't my area.