* What constitutes "proper grammar"? Grammar rules can be complex and vary depending on context, formality, and even regional dialects.
* Subjectivity of evaluation: What one person considers a grammatical error, another might see as acceptable variation.
* Lack of comprehensive data: There are no large-scale studies specifically focused on measuring grammar proficiency across the entire US population.
Instead of focusing on a percentage, it's more helpful to acknowledge that:
* Grammar is a spectrum: Everyone makes mistakes, and fluency in grammar is a spectrum, not a binary "correct" or "incorrect" state.
* Improving grammar is a lifelong process: Even native speakers continue to learn and refine their grammar skills throughout their lives.
* Focus on communication: The most important aspect is communicating effectively and being understood, regardless of minor grammatical imperfections.
Instead of trying to measure a percentage, it's more productive to focus on promoting communication skills and fostering a positive attitude towards language learning.