Hi Yann,

This field is actually a calculated value - from the 'Total Days to Pay' and the 'Total Invs Paid' fields - which is why you cannot see a particalar field called 'Avg days to pay'. This number is not directly stored in the data.

You will need to create a calculated column that does the same calculation:
IF( {Total Invs Paid}<>0 , {Total Days to Pay}/{Total Invs Paid} , 0 )