Estimate time for a grammar review.
Grammar review time is dominated by resolving issues, not finding them. An automated tool scans a 3000-word document in seconds but catches only around three quarters of genuine errors while adding roughly 30 percent false positives you still have to read and dismiss. Manual review catches more but requires reading at a slow deliberate pace of about 1500 words an hour. The combination catches the most and is what most careful writers do, but the arithmetic is honest about the cost: at 25 seconds per flagged item, thirty-six items is fifteen minutes of decisions on top of the reading.
Issues flagged
Items = words / 1000 x error density x method recall x (1 + false positive rate)
Total time
Minutes = scan time + items x seconds per fix / 60
False positives. Grammar checkers flag legitimate constructions — passive voice used deliberately, technical terminology, deliberate sentence fragments — at a rate of roughly 30 percent of true errors. Each one still costs you the time to read and dismiss it, which is real work even though nothing changes.
For anything assessed or published, yes. Automated tools miss the errors that matter most: wrong-but-real words, subject-verb agreement across long clauses, and inconsistent terminology. Those are the errors a reader notices, and they need a human pass to catch.