The population was categorized by mother tongue i.e. the primary language in the following categories: Polish, Ukrainian, Ruthenian (i.e. Rusyn), Belarusian, Russian, Lithuanian, German, Yiddish, Hebrew, Local, Other, and Not Declared.

[3][8] However, a higher percent of Jews by religion - over 25% - considered themselves to be ethnically (or in terms of national identity) Poles, according to the previous census of 1921.

This situation created a difficulty in establishing the true number of ethnic non-Polish citizens of Poland.

[11] After World War II in Soviet bloc countries the interpretation of the census was used for political purposes, to underline the officially-supported thesis that pre-war Poland incorporated areas where the non-Polish population made up the majority of inhabitants.

For this purpose some authors combined all non-Polish speakers in South-Eastern Poland (namely Ukrainians, Belarusians, Rusyns, Hutsuls, Lemkos, Boykos and Poleszuks) into one category of "Ruthenians").

[12] In fact, the census had counted speakers of Belarusian, Ukrainian, Russian, and Ruthenian languages as separate categories[6] Some authors contend that the change in questions asked by the census officials was due to the Polish government's wish to minimise the presence of minorities[3][13][14] and represented an attempt to maximize the effects of a decade of educational policies stressing the Polish language.

[17] Another English language account stated that he admitted "that officials had been directed to undercount minorities, especially those in the eastern provinces".