; (C) a description of progress against the plan of action and milestones required under subsection (a)(3)(C), including qualitative descriptions of significant challenges and limitations hindering data improvement efforts; and (D) observations from the Administrator for Federal Procurement Policy regarding the respective advantages and disadvantages of various approaches for establishing and using outcome-oriented contracting metrics and goals. 5. Government accountability office reporting Not later than 180 days after receiving the interim and final reports required under section 4, the Comptroller General of the United States shall provide the Comptroller General’s independent observations on the report to the appropriate congressional committees.