Citation required as this is heavily dependent upon the purpose of the home schooling and the resources of the parents.

Anecdata: My cousin was not doing well in 4th grade. So, they pulled her out and her grandmother taught her for 8 hours a day, 5 days a week, every single day.

By the time she came back to school in 5th grade she was doing extremely well and hyper motivated to never go through that experience again. :)

She was, however, the exception, not the rule. So much so that a lot of teachers complimented her and her family on the outcome. Her family had no agenda other than "fix her education". Most homeschoolers do not bring to bear that amount of resource and attention, and they have quite a bit poorer results.