How bad are these scores? Very bad. If you know half the letters in the alphabet, we don’t call you “half literate.” We correctly call you illiterate.

The general pattern is that grown-ups have shockingly little academic knowledge. College graduates know about what you’d expect high school graduates to know; high school graduates know about what you’d expect dropouts to know; dropouts know next to nothing. This doesn’t mean that these students never knew more; it just means that only a tiny fraction of what they learn durably stays in their heads.