This is one of my first custom projects where I combined both detailed census data (Lithuania - 2011, Poland - 2001, Latvia - 2011, Belarus - 2009, Russia - 2010) and available secondary sources which deal with the distribution of nationalities around the border areas.

The most difficult case was Belarus. The 2009 census data was available only on highly aggregated level. Any earlier studies I could work from were from 1950s and 1960s. However, combining these with the aggregated census data allowed me to assume which areas should still have the majority of Poles living in Belarus.

The issue of language vs ethnicity is another topic of discussion. Many Vilnius area Poles speak the "po prostu" language which is closer to Belarusian rather than Polish vernacular. Nevertheless, that's up for future projects to explore upon.