Readability formulas emerged in the 1940s and 1950s when educators needed objective tools to match textbooks to student reading levels. They measure two primary factors strongly correlated with reading difficulty: sentence length (longer sentences demand more working memory) and word complexity (multi-syllabic words are harder to decode). Despite being 70+ years old, these formulas remain valid predictors of comprehension difficulty.
Why Plain English Matters
Research confirms that average Americans read at approximately Grade 8, yet most patient education materials score at Grade 12–18. Consumer contracts routinely exceed Grade 20. The US Plain Writing Act (2010) requires federal agencies to write in plain language for public documents. Studies consistently show that lowering reading level increases comprehension, reduces errors, and improves user satisfaction — without making content feel "dumbed down."
Choosing the Right Grade Level
The goal is always to match complexity to audience. Children's books: Grade 2–4. Marketing copy: Grade 6–8. News: Grade 8–10. Business reports: Grade 10–12. Academic papers: Grade 14+. Legal documents benefit most from simplification — most can be rewritten at Grade 10–12 without losing precision. When in doubt, write simpler: you never lose a reader by being too clear.