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: .
[['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%