QGIS » QGIS Application

I have a basic QGIS problem that some of my students encountered in spring (version 2.6) but now hits me as well (version 2.8).
When I try to either convert a string attribute to double or when I just try to calculate a rate by dividing two integer variables in by shapefile table, the result initially is displayed correctly but when I then save the edits, the column automatically reverts to integer.
This happened to me now with different shapefiles before or after a join tables operation.
If I explicitly create an empty new column of type double and save it, it gets indeed saved as double (quitting QGIS, restarting and reloading the file). But even then, when I use the field calculator to recalculate the values of that double field, the results initially show as floating point values but after a screen refresh show only as whole numbers - although now the filed type remains double.
I have not found anything on the web that suggests that others ran into such a problem - other than my students from this spring that is.
Since Excel doesn't deal with dBase files, I now resort for myself to working on the dBase part of the shapefile in LibreOffice - but this is not a tenable solution for beginning students once our semester starts this September.