for the estimates of our oil sands mining reserves has 33 years of experience in petroleum engineering and has conducted surface mineable oil sands evaluations since 1986 . he is a member of spe , having served as regional director from 1998 through 2001 and is a registered practicing professional engineer in the province of alberta . audits of estimates third-party consultants are engaged to provide independent estimates for fields that comprise 80 percent of our total proved reserves over a rolling four-year period for the purpose of auditing the in-house reserve estimates . we met this goal for the four-year period ended december 31 , 2011 . we established a tolerance level of 10 percent such that initial estimates by the third-party consultants are accepted if they are within 10 percent of our internal estimates . should the third-party consultants 2019 initial analysis fail to reach our tolerance level , both our team and the consultants re-examine the information provided , request additional data and refine their analysis if appropriate . this resolution process is continued until both estimates are within 10 percent . this process did not result in significant changes to our reserve estimates in 2011 or 2009 . there were no third-party audits performed in 2010 . during 2011 , netherland , sewell & associates , inc . ( 201cnsai 201d ) prepared a certification of december 31 , 2010 reserves for the alba field in equatorial guinea . the nsai summary report is filed as an exhibit to this annual report on form 10-k . the senior members of the nsai team have over 50 years of industry experience between them , having worked for large , international oil and gas companies before joining nsai . the team lead has a master of science in mechanical engineering and is a member of spe . the senior technical advisor has a bachelor of science degree in geophysics and is a member of the society of exploration geophysicists , the american association of petroleum geologists and the european association of geoscientists and engineers . both are licensed in the state of texas . ryder scott company ( 201cryder scott 201d ) performed audits of several of our fields in 2011 and 2009 . their summary report on audits performed in 2011 is filed as an exhibit to this annual report on form 10-k . the team lead for ryder scott has over 20 years of industry experience , having worked for a major international oil and gas company before joining ryder scott . he has a bachelor of science degree in mechanical engineering , is a member of spe and is a registered professional engineer in the state of texas . the corporate reserves group also performs separate , detailed technical reviews of reserve estimates for significant fields that were acquired recently or for properties with other indicators such as excessively short or long lives , performance above or below expectations or changes in economic or operating conditions . changes in proved undeveloped reserves as of december 31 , 2011 , 395 mmboe of proved undeveloped reserves were reported , a decrease of 10 mmboe from december 31 , 2010 . the following table shows changes in total proved undeveloped reserves for 2011: .

Table: beginning of year 405 revisions of previous estimates 15 improved recovery 1 purchases of reserves in place 91 extensions discoveries and other additions 49 transfer to proved developed -166 ( 166 ) end of year 395

significant additions to proved undeveloped reserves during 2011 include 91 mmboe due to acreage acquisition in the eagle ford shale , 26 mmboe related to anadarko woodford shale development , 10 mmboe for development drilling in the bakken shale play and 8 mmboe for additional drilling in norway . additionally , 139 mmboe were transferred from proved undeveloped to proved developed reserves due to startup of the jackpine upgrader expansion in canada . costs incurred in 2011 , 2010 and 2009 relating to the development of proved undeveloped reserves , were $ 1107 million , $ 1463 million and $ 792 million . projects can remain in proved undeveloped reserves for extended periods in certain situations such as behind-pipe zones where reserves will not be accessed until the primary producing zone depletes , large development projects which take more than five years to complete , and the timing of when additional gas compression is needed . of the 395 mmboe of proved undeveloped reserves at year end 2011 , 34 percent of the volume is associated with projects that have been included in proved reserves for more than five years . the majority of this volume is related to a compression project in equatorial guinea that was sanctioned by our board of directors in 2004 and is expected to be completed by 2016 . performance of this field has exceeded expectations , and estimates of initial dry gas in place increased by roughly 10 percent between 2004 and 2010 . production is not expected to experience a natural decline from facility-limited plateau production until 2014 , or possibly 2015 . the timing of the installation of compression is being driven by the reservoir performance. .

Question: by how much did total proved undeveloped reserves decrease during 2011?
Answer: -2.5%
Calculation: divide(subtract(395, 405), 405)