what is patient 13837's monthly average factor viii level since 2103?
select avg(labevents.valuenum) from labevents where labevents.hadm_id in ( select admissions.hadm_id from admissions where admissions.subject_id = 13837 ) and labevents.itemid in ( select d_labitems.itemid from d_labitems where d_labitems.label = 'factor viii' ) and strftime('%y',labevents.charttime) >= '2103' group by strftime('%y-%m',labevents.charttime)