Why is sampling not an option for the depression measure like it is for other measures? 

 Sampling is not permissible because the measure tracks patients prospectively from the date they meet the index criteria over a six-month period. If a sampling methodology were used, patients included in a sample in the middle or later in the six-month period may not be included in a subsequent sample, excluding many of their follow-up PHQ-9 scores. 

For this reason it is required that the medical group submit data for the entire population of patients with major depression or dysthymia.