proved reserves can be added as expansions are permitted , funding is approved and certain stipulations of the joint venture agreement are satisfied . the following table sets forth changes in estimated quantities of net proved bitumen reserves for the year 2008 . estimated quantities of proved bitumen reserves ( millions of barrels ) 2008 .

Table: ( millions of barrels ) 2008 beginning of year 421 revisions ( a ) -30 ( 30 ) extensions discoveries and additions 6 production -9 ( 9 ) end of year 388

( a ) revisions were driven primarily by price and the impact of the new royalty regime discussed below . the above estimated quantity of net proved bitumen reserves is a forward-looking statement and is based on a number of assumptions , including ( among others ) commodity prices , volumes in-place , presently known physical data , recoverability of bitumen , industry economic conditions , levels of cash flow from operations , and other operating considerations . to the extent these assumptions prove inaccurate , actual recoveries could be different than current estimates . for a discussion of the proved bitumen reserves estimation process , see item 7 . management 2019s discussion and analysis of financial condition and results of operations 2013 critical accounting estimates 2013 estimated net recoverable reserve quantities 2013 proved bitumen reserves . operations at the aosp are not within the scope of statement of financial accounting standards ( 201csfas 201d ) no . 25 , 201csuspension of certain accounting requirements for oil and gas producing companies ( an amendment of financial accounting standards board ( 201cfasb 201d ) statement no . 19 ) , 201d sfas no . 69 , 201cdisclosures about oil and gas producing activities ( an amendment of fasb statements 19 , 25 , 33 and 39 ) , 201d and securities and exchange commission ( 201csec 201d ) rule 4-10 of regulation s-x ; therefore , bitumen production and reserves are not included in our supplementary information on oil and gas producing activities . the sec has recently issued a release amending these disclosure requirements effective for annual reports on form 10-k for fiscal years ending on or after december 31 , 2009 , see item 7 . management 2019s discussion and analysis of financial condition and results of operations 2013 accounting standards not yet adopted for additional information . prior to our acquisition of western , the first fully-integrated expansion of the existing aosp facilities was approved in 2006 . expansion 1 , which includes construction of mining and extraction facilities at the jackpine mine , expansion of treatment facilities at the existing muskeg river mine , expansion of the scotford upgrader and development of related infrastructure , is anticipated to begin operations in late 2010 or 2011 . when expansion 1 is complete , we will have more than 50000 bpd of net production and upgrading capacity in the canadian oil sands . the timing and scope of future expansions and debottlenecking opportunities on existing operations remain under review . during 2008 , the alberta government accepted the project 2019s application to have a portion of the expansion 1 capital costs form part of the muskeg river mine 2019s allowable cost recovery pool . due to commodity price declines in the year , royalties for 2008 were one percent of the gross mine revenue . commencing january 1 , 2009 , the alberta royalty regime has been amended such that royalty rates will be based on the canadian dollar ( 201ccad 201d ) equivalent monthly average west texas intermediate ( 201cwti 201d ) price . royalty rates will rise from a minimum of one percent to a maximum of nine percent under the gross revenue method and from a minimum of 25 percent to a maximum of 40 percent under the net revenue method . under both methods , the minimum royalty is based on a wti price of $ 55.00 cad per barrel and below while the maximum royalty is reached at a wti price of $ 120.00 cad per barrel and above , with a linear increase in royalty between the aforementioned prices . the above discussion of the oil sands mining segment includes forward-looking statements concerning the anticipated completion of aosp expansion 1 . factors which could affect the expansion project include transportation logistics , availability of materials and labor , unforeseen hazards such as weather conditions , delays in obtaining or conditions imposed by necessary government and third-party approvals and other risks customarily associated with construction projects . refining , marketing and transportation refining we own and operate seven refineries in the gulf coast , midwest and upper great plains regions of the united states with an aggregate refining capacity of 1.016 million barrels per day ( 201cmmbpd 201d ) of crude oil . during 2008 .

Question: by how much did proved bitumen reserves decrease during 2008?
Answer: -7.8%
Calculation: divide(subtract(388, 421), 421)