> Some of these high county-level percentages stem from high populations of immigrants, whose first language is not English. The PIAAC only assesses English literacy, though its background questionnaire is given in English and Spanish.

If you look at the map (and read the article), it's fairly obvious that they are NOT adjusting for non-English first language speakers. This is partly on purpose since it's those demographics that need the most assistance and funding to learn English. However, it's really disappointing that this data is used to make statements and titles regarding people's "literacy" or reading comprehension when it's specifically testing a single language.