By default, delivery date and statistical delivery date will be the same when you create your PO. But vendor evaluation uses statistical delivery date in calculating on time delivery score.

Change in statistical delivery date happen when, vendor initially promised a date and much later change the date. You can choose to change delivery date and statistical delivery date thus vendor evaluation for this vendor will be "on-time".

If you otherwise decide to maintain original statistical delivery date, you will evaluate your vendor as per initial delivery date thus your vendor will score lower in "on time" criteria.
 

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