Calculating overall TRI value using QGIS

TRI algorithm was made according to this science paper "Riley, S.J., De Gloria, S.D., Elliot, R. (1999): A Terrain Ruggedness that Quantifies Topographic Heterogeneity. Intermountain Journal of Science". First, you have to know which pixel size you have? If your pixel size 20x20 then you have some calculations. TRI Riley classification is based on 1km x 1km pixel size, and you have to customize your map. In SAGA GIS terrain ruggedness index algorithm, you should use "Search Radius = 25" (1000/20 = 50 = edge length; because a radius is: 50/2 = 25). (Better results are obtained with SAGA GIS software). After that, you have to convert Riley classification to yours. Each class, you should to reclassify like this: First class according to Riley is 0-80, and you should use calculation (80:1000=x:20), you will get 1,6 value which is discribed as "Level", and second is 2,32 ="Nearly Level" ect...

The answer to my question was to run Zonal Statistics for mean TRI. That would allow me to report a mean value for the entire sampled area.