Financial performance measurement.

Dear John, How are you? I hope you are doing well, I have a question regarding average collection period equals to (account receivable divided by credit sales) *365 days OK but why don't we use collection amount rather than account receivable balance? Is this ratio which you explained in your informative lecture take into consideration the balance of write off of receivables. The last one is shall we take the gross amount of account receivable or net realizable value. Thank you in advance.