Based on the data your provided
SupplierCode, DateDue, ratioOnTime
317, 11/1/2016, .7
317, 12/1/2016, .5
317, 1/1/2017, .8
317, 2/1/2017, .9
317, 3/1/2017, .6
317, 5/1/2017, .2
444, 4/1/2017, .6
444, 3/1/2017, .5
here is the result:
SupplierCode, DateDue, avg(ratioOnTime )
317,1/1/2017/,.67...