The number of patients who've tried out retail clinics turns out to be modest, say the authors of a study from the Center for Studying Health System Change. Only 3.4 million American families, or 2.3%, had used a retail clinic as of 2007, according to survey results. The study's authors noted that retail clinics have turned out to be more complex and costly to operate than expected, and some physician practices are responding by extending their own office hours and doing more same-day scheduling.