what is the yearly average amount of phenytoin patient 23475 has until 01/2105?
select avg(labevents.valuenum) from labevents where labevents.hadm_id in ( select admissions.hadm_id from admissions where admissions.subject_id = 23475 ) and labevents.itemid in ( select d_labitems.itemid from d_labitems where d_labitems.label = 'phenytoin' ) and strftime('%y-%m',labevents.charttime) <= '2105-01' group by strftime('%y',labevents.charttime)