Maybe I'm the odd duck but I wouldn't have any policy for unexcused absences or ever require a doctors note. If there's an employee with attendance issues, could you speak with her about it and if it doesn't remedy itself, look to replace her? Then during the interview or on boarding process, explain your expectations for attendance. There's a lot of debate on doctors notes for adults. Generally speaking, most of us know when we're too sick to go in but don't need to go to the dr. Hopefully an employer-to-employee trusting relationship can be established
I think it's lack of work ethic. If it's an issue with many employees, over a period of time, and they start out really great and then decline - consider looking at yourself or the environment. NOT saying you're the problem with this example! Just speaking from experience with a toxic workplace where our manager couldn't understand why there was high turnover, poor attendance, poor performance, etc.