Literacy rates in the U.S.

 I saw the following tweet from Jorie Graham, which claims that 21% of U.S. adults are illiterate and 54% of adults have a literacy level below 6th grade and thought, "can that really be true?" and it became today's mini-rabbit hole.  

Sadly, Jorie gives no sources for these numbers, but with a quick check of the Wikipedia page on Literacy in the United States we find in the summary:

• "over 20% of adult Americans have a literacy proficiency at or below Level 1.  Adults in this range have difficulty using or understanding print materials."  So not completely illiterate, but likely unable to get through this blog post. 
• "54% of adults in the United States lack English literacy proficiency."  This number comes from a Gallup report that defines literacy proficiency as scoring Level 3 or higher on the PIAAC.  

Okay, so this raises questions:

What is the PIAAC?  The the Programme for the International Assessment of Adult Competencies, and has literacy, numeracy, and problem-solving assessments. 

What is the distribution of Americans at each level of the PIAAC?  There's a short report from the National Center for Education Statistics (NCES) that 

What does each level signify?  The literacy proficieny levels section of this NCES page has some long descriptions; the competence groups section of the PIAAC Wikipedia page has useful summaries.

I looked at the literacy sample items section of this NCES page in order to try to get a more practical feel for the kinds of skills readers are supposed to have at each level.  The descriptions of the levels and the sample items shown on the NCES page don't seem to match up in difficulty to me; I'm not sure what to make of that. Googling for "piaac literacy sample items" turns up similar sample materials, but just as examples of test items without matching them to a particular level.  I'm hoping that a well-meaning person at NCES mapped the sample items to levels badly.

How are non-native English speakers handled?  Not very well.  The above reports ignore the issue entirely, and while there is another report on English Literacy and Language Minorities in the United States, trying to apply the information in here to get a succinct set of results is too much work for a weekend mini-rabbit hole.

The following table summarizes what I've looked at, and yes, it's terrifying that half the adult population in the U.S. lacks literacy proficiency.