There are not that many absolutes when it comes to employee versus independent contractor, but "ways and means" of doing the job, and the requiring of a certain number of, or a minimum number of working days are two of them which instantly turns the independent contractor into an employee.
An independent contractor can set their own hours, come and go as they please, and set their own work days. Anything that restricts or sets a quota for hours or work days, infringes upon the independent contractor status, making them an employee.
Also, requiring a minimum level of productivity, as Greg note, is "direction and control", and falls under "ways and means', which makes them instant employees.