indicate the yearly average volume of mediastinal that patient 15209 has had since 03/2103.
select avg(outputevents.value) from outputevents where outputevents.icustay_id in ( select icustays.icustay_id from icustays where icustays.hadm_id in ( select admissions.hadm_id from admissions where admissions.subject_id = 15209 ) ) and outputevents.itemid in ( select d_items.itemid from d_items where d_items.label = 'mediastinal' and d_items.linksto = 'outputevents' ) and strftime('%y-%m',outputevents.charttime) >= '2103-03' group by strftime('%y',outputevents.charttime)