Q:

Missing Options in r.neighbours

I tried to use the GRASS GIS tool r.neighbors from QGIS with the method "quart1" and "quart3". But it is not possible to select one of these methods. In GRASS GIS itself it worked well.
Any ideas for a workaround for this issue in QGIS?

A:

You could modify the parameters stored in the text file "r.neighbors.txt" to include those which are missing. This can be found in your QGIS directory, e.g.:
/QGIS 2.18/apps/qgis-ltr/python/plugins/processing/algs/grass7/description/r.neighbors.txt

When you open this file, it will probably look something like:
r.neighbors
Makes each cell category value a function of the category values assigned to the cells around it
Raster (r.*)
ParameterRaster|input|Input raster layer|False
ParameterSelection|method|Neighborhood operation|average;median;mode;minimum;maximum;stddev;sum;variance;diversity;interspersion
...

The line beginning ParameterSelection contains the list of  input methods. Just add the remaining ones separated by a semi-colon (e.g. ;quart1;quart3;perc90;quantile which were obtained from the r.neighbours manual):
r.neighbors
Makes each cell category value a function of the category values assigned to the cells around it
Raster (r.*)
ParameterRaster|input|Input raster layer|False
ParameterSelection|method|Neighborhood operation|average;median;mode;minimum;maximum;stddev;sum;variance;diversity;interspersion;quart1;quart3;perc90;quantile
...

Then save the file (note that you may need rights to save the file. If an error occurs, you can just save the file on desktop, delete the original file then move the edited version in its place.)
Restart QGIS and the options should be available which you can then use:

