Restricted Stock and Restricted Stock Units.
Employees may be granted restricted (non-vested) stock and/or restricted stock units without cost to them.
The restricted stock and restricted stock units generally vest five years after the date of grant, except for certain bonus grants, and as defined in individual grant agreements.
Upon vesting of restricted stock, shares of Common Stock are released to the employee.
Upon vesting, restricted stock units are converted into shares of Common Stock and released to the employee.
Stock-based compensation expense related to restricted stock and restricted stock units totaled $60 million, $69 million and $74 million for the years ended December 31, 2016, 2015 and 2014, respectively.
The following table sets forth the restricted stock and restricted stock unit transactions for the years ended December 31, 2016, 2015 and 2014 (shares and units in thousands):
## Table 0 ##
(1) The total intrinsic value of restricted stock and restricted stock units released during the years ended December 31, 2016, 2015 and 2014 was $124 million, $109 million and $291 million, respectively.
The intrinsic value is based upon the closing price of EOG's common stock on the date restricted stock and restricted stock units are released.
(2) The total intrinsic value of restricted stock and restricted stock units outstanding at December 31, 2016 and 2015 was approximately $401 million and $347 million, respectively.
At December 31, 2016, unrecognized compensation expense related to restricted stock and restricted stock units totaled $153 million.
Such unrecognized expense will be recognized on a straight-line basis over a weighted average period of 2.5 years.
Performance Units and Performance Stock.
EOG has granted performance units and/or performance stock (Performance Awards) to its executive officers annually since 2012.
As more fully discussed in the grant agreements, the performance metric applicable to these performance-based grants is EOG's total shareholder return over a three-year performance period relative to the total shareholder return of a designated group of peer companies (Performance Period).
Upon the application of the performance multiple at the completion of the Performance Period, a minimum of 0% and a maximum of 200% of the Performance Awards granted could be outstanding.
Subject to the termination provisions set forth in the grant agreements and the applicable performance multiple, the grants of Performance Awards will generally "cliff" vest five years from the date of grant.
The fair value of the Performance Awards is estimated using a Monte Carlo simulation.
Stock-based compensation expense related to the Performance Awards totaled $11 million, $5 million and $9 million for the years ended December 31, 2016, 2015 and 2014, respectively.
At December 31, 2015, 405,000 Performance Awards were outstanding.
Upon completion of the Performance Period for the Performance Awards granted in 2012, a performance multiple of 200% was applied to the 2012 grants resulting in an additional grant of 142,556 Performance Awards in February 2016.
During the twelve-month period ended December 31, 2016, a total of 131,750 Performance Awards were granted.
A total of 134,016 Performance Awards were released during the twelve months ended December 31, 2016, with a total intrinsic value of $9.7 million, based upon the closing price of EOG's common stock on the release date.
Upon the application of the performance multiple at the completion of the remaining Performance Periods, a minimum of 151,096 and a maximum of 939,484 Performance Awards could be outstanding.
There were 545,290 Performance Awards outstanding as of December 31, 2016.
The total intrinsic value of Performance Awards outstanding at December 31, 2016 was $55.1 million.
Upon completion of the performance period for the Performance Awards granted in 2013, a performance multiple of 200% was applied to the 2013 grants resulting in an additional grant of 118,834 Performance Awards in February 2017.
The principal components of EOG's rollforward of valuation allowances for deferred income tax assets for the years indicated below were as follows (in thousands):
## Table 1 ##
(1) Increase in valuation allowance related to the generation of tax NOLs and other deferred tax assets.
(2) Decrease in valuation allowance associated with adjustments to certain deferred tax assets and their related allowance.
(3) Represents dispositions, revisions and/or foreign exchange rate variances and the effect of statutory income tax rate changes.
The United Kingdom operations were sold in the fourth quarter of 2018.
The Argentina operations were sold in the third quarter of 2016.
As of December 31, 2018, EOG had state income tax NOLs being carried forward of approximately $1.8 billion, which, if unused, expire between 2019 and 2037.
EOG also has Canadian NOLs of $183 million which can be carried forward 20 years.
As described above, these NOLs as well as other less significant future tax benefits, have been evaluated for the likelihood of utilization, and valuation allowances have been established for the portion of these deferred income tax assets that do not meet the “more likely than not” threshold.
The balance of unrecognized tax benefits at December 31, 2018, was $29 million, resulting from the tax treatment of its research and experimental expenditures related to certain innovations in its horizontal drilling and completion projects, of which $12 million may potentially have an earnings impact.
EOG records interest and penalties related to unrecognized tax benefits to its income tax provision.
Currently $2 million of interest has been recognized in the Consolidated Statements of Income (Loss) and Comprehensive Income (Loss).
EOG does not anticipate that the amount of the unrecognized tax benefits will change materially during the next twelve months.
EOG and its subsidiaries file income tax returns and are subject to tax audits in the U. S.  and various state, local and foreign jurisdictions.
EOG's earliest open tax years in its principal jurisdictions are as follows: U. S.  federal (2016), Canada (2014), Trinidad (2013) and China (2008).
EOG's foreign subsidiaries' undistributed earnings are not considered to be permanently reinvested outside of the U. S.  Accordingly, EOG may be required to accrue certain U. S.  federal, state, and foreign deferred income taxes on these undistributed earnings as well as on any other outside basis differences related to its investments in these subsidiaries.
As of December 31, 2018, EOG has cumulatively recorded $23 million of deferred foreign income taxes for withholdings on its undistributed foreign earnings.
Additionally, for tax years beginning in 2018 and later, EOG's foreign earnings may be subject to the U. S.  federal "global intangible low-taxed income" (GILTI) inclusion.
EOG records any GILTI tax as a period expense.7.
Employee Benefit Plans Stock-Based Compensation During 2018, EOG maintained various stock-based compensation plans as discussed below.
EOG recognizes compensation expense on grants of stock options, SARs, restricted stock and restricted stock units, performance units and grants made under the EOG Resources, Inc.
Employee Stock Purchase Plan (ESPP).
Stock-based compensation expense is calculated based upon the grant date estimated fair value of the awards, net of forfeitures, based upon EOG's historical employee turnover rate.
Compensation expense is amortized over the shorter of the vesting period or the period from date of grant until the date the employee becomes eligible to retire without company approval.
The following table sets forth the amounts and classification of EOG's outstanding derivative financial instruments at December 31, 2018 and 2017, respectively.
Certain amounts may be presented on a net basis on the consolidated financial statements when such amounts are with the same counterparty and subject to a master netting arrangement (in millions):
## Table 2 ##
(1) The current portion of Liabilities from Price Risk Management Activities consists of gross liabilities of $55 million, partially offset by gross assets of $5 million, at December 31, 2017.  Credit Risk.
Notional contract amounts are used to express the magnitude of a financial derivative.
The amounts potentially subject to credit risk, in the event of nonperformance by the counterparties, are equal to the fair value of such contracts (see Note 13).
EOG evaluates its exposure to significant counterparties on an ongoing basis, including those arising from physical and financial transactions.
In some instances, EOG renegotiates payment terms and/or requires collateral, parent guarantees or letters of credit to minimize credit risk.
At December 31, 2018, EOG's net accounts receivable balance related to United States hydrocarbon sales included three receivable balances, each of which accounted for more than 10% of the total balance.
The receivables were due from three petroleum refinery companies.
The related amounts were collected during early 2019.
At December 31, 2017, EOG's net accounts receivable balance related to United States, Canada and United Kingdom hydrocarbon sales included two receivable balances, each of which accounted for more than 10% of the total balance.
The receivables were due from two petroleum refinery companies.
The related amounts were collected during early 2018.
In 2018 and 2017, all natural gas from EOG's Trinidad operations was sold to the National Gas Company of Trinidad and Tobago Limited and its subsidiary.
In 2018, all crude oil and condensate from EOG's Trinidad operations was sold to the Petroleum Company of Trinidad and Tobago Limited and its successor, Heritage Petroleum Company Limited.
In 2017, all crude oil and condensate from EOG's Trinidad operations was sold to the Petroleum Company of Trinidad and Tobago Limited; and in 2018 and 2017, all natural gas from EOG's China operations was sold to Petrochina Company Limited.
All of EOG's derivative instruments are covered by International Swap Dealers Association Master Agreements (ISDAs) with counterparties.
The ISDAs may contain provisions that require EOG, if it is the party in a net liability position, to post collateral when the amount of the net liability exceeds the threshold level specified for EOG's then-current credit ratings.
In addition, the ISDAs may also provide that as a result of certain circumstances, including certain events that cause EOG's credit ratings to become materially weaker than its then-current ratings, the counterparty may require all outstanding derivatives under the ISDAto be settled immediately.
See Note 13 for the aggregate fair value of all derivative instruments that were in a net liability position at December 31, 2017.
EOG had no collateral posted and held no collateral at December 31, 2018 and 2017.
Substantially all of EOG's accounts receivable at December 31, 2018 and 2017 resulted from hydrocarbon sales and/or joint interest billings to third-party companies, including foreign state-owned entities in the oil and gas industry.
This concentration of customers and joint interest owners may impact EOG's overall credit risk, either positively or negatively, in that these entities may be similarly affected by changes in economic or other conditions.
In determining whether or not to require collateral or other credit enhancements from a customer or joint interest owner, EOG typically analyzes the entity's net worth, cash flows, earnings and credit ratings.
Receivables are generally not collateralized.
During the three-year period ended December 31, 2018, credit losses incurred on receivables by EOG have been immaterial.
EOG RESOURCES, INC.
SUPPLEMENTAL INFORMATION TO CONSOLIDATED FINANCIAL STATEMENTS (Continued)
## Table 3 ##
(1) Other International includes EOG's United Kingdom, China, Canada and Argentina operations.
The United Kingdom operations were sold in the fourth quarter of 2018.
The Argentina operations were sold in the third quarter of 2016.
(2) Thousand barrels or thousand barrels of oil equivalent, as applicable; oil equivalents include crude oil and condensate, NGLs and natural gas.
Oil equivalents are determined using a ratio of 1.0 barrel of crude oil and condensate or NGLs to 6.0 thousand cubic feet of natural gas.
(3) Billion cubic feet.

<table><tr><td></td><td colspan="2">2016</td><td colspan="2">2015</td><td colspan="2">2014</td></tr><tr><td></td><td>Number ofShares andUnits</td><td>WeightedAverageGrant DateFair Value</td><td>Number ofShares andUnits</td><td>WeightedAverageGrant DateFair Value</td><td>Number ofShares andUnits</td><td>WeightedAverageGrant DateFair Value</td></tr><tr><td>Outstanding at January 1</td><td>4,908</td><td>$70.35</td><td>5,394</td><td>$64.39</td><td>7,358</td><td>$49.54</td></tr><tr><td>Granted</td><td>853</td><td>88.01</td><td>1,044</td><td>77.94</td><td>1,132</td><td>98.72</td></tr><tr><td>Released<sup>-1</sup></td><td>-1,465</td><td>53.95</td><td>-1,331</td><td>51.52</td><td>-2,761</td><td>105.24</td></tr><tr><td>Forfeited</td><td>-334</td><td>77.29</td><td>-199</td><td>74.56</td><td>-335</td><td>62.55</td></tr><tr><td>Outstanding at December 31<sup>-2</sup></td><td>3,962</td><td>79.63</td><td>4,908</td><td>70.35</td><td>5,394</td><td>64.39</td></tr></table>
<table><tr><td></td><td>2018</td><td>2017</td><td>2016</td></tr><tr><td>Beginning Balance</td><td>$466,421</td><td>$383,221</td><td>$506,127</td></tr><tr><td>Increase<sup>-1</sup></td><td>23,062</td><td>67,333</td><td>37,221</td></tr><tr><td>Decrease<sup>-2</sup></td><td>-26,219</td><td>-13,687</td><td>-12,667</td></tr><tr><td>Other<sup>-3</sup></td><td>-296,122</td><td>29,554</td><td>-147,460</td></tr><tr><td>Ending Balance</td><td>$167,142</td><td>$466,421</td><td>$383,221</td></tr></table>
<table><tr><td></td><td></td><td colspan="2">Fair Value at December 31,</td></tr><tr><td>Description</td><td>Location on Balance Sheet</td><td>2018</td><td>2017</td></tr><tr><td>Asset Derivatives</td><td></td><td></td><td></td></tr><tr><td>Crude oil and natural gas derivative contracts -</td><td></td><td></td><td></td></tr><tr><td>Current portion</td><td>Assets from Price Risk Management Activities</td><td>$24</td><td>$8</td></tr><tr><td>Noncurrent portion</td><td>Other Assets</td><td>—</td><td>—</td></tr><tr><td>Liability Derivatives</td><td></td><td></td><td></td></tr><tr><td>Crude oil and natural gas derivative contracts -</td><td></td><td></td><td></td></tr><tr><td>Current portion</td><td>Liabilities from Price Risk Management Activities<sup>-1</sup></td><td>$—</td><td>$50</td></tr><tr><td>Noncurrent portion</td><td>Other Liabilities</td><td>—</td><td>7</td></tr></table>
<table><tr><td></td><td>UnitedStates</td><td>Trinidad</td><td>OtherInternational<sup>-1</sup></td><td>Total</td></tr><tr><td>Natural Gas (Bcf)<sup>(3)</sup></td><td></td><td></td><td></td><td></td></tr><tr><td>Net proved reserves at December 31, 2015</td><td>3,489.8</td><td>316.6</td><td>19.5</td><td>3,825.9</td></tr><tr><td>Revisions of previous estimates</td><td>298.4</td><td>29.5</td><td>5.2</td><td>333.1</td></tr><tr><td>Purchases in place</td><td>91.5</td><td>—</td><td>—</td><td>91.5</td></tr><tr><td>Extensions, discoveries and other additions</td><td>202.1</td><td>59.9</td><td>—</td><td>262.0</td></tr><tr><td>Sales in place</td><td>-752.0</td><td>—</td><td>—</td><td>-752.0</td></tr><tr><td>Production</td><td>-308.6</td><td>-125.1</td><td>-8.9</td><td>-442.6</td></tr><tr><td>Net proved reserves at December 31, 2016</td><td>3,021.2</td><td>280.9</td><td>15.8</td><td>3,317.9</td></tr><tr><td>Revisions of previous estimates</td><td>602.8</td><td>-27.4</td><td>8.6</td><td>584.0</td></tr><tr><td>Purchases in place</td><td>4.8</td><td>—</td><td>—</td><td>4.8</td></tr><tr><td>Extensions, discoveries and other additions</td><td>619.3</td><td>174.2</td><td>35.9</td><td>829.4</td></tr><tr><td>Sales in place</td><td>-56.4</td><td>—</td><td>—</td><td>-56.4</td></tr><tr><td>Production</td><td>-293.2</td><td>-114.3</td><td>-9.1</td><td>-416.6</td></tr><tr><td>Net proved reserves at December 31, 2017</td><td>3,898.5</td><td>313.4</td><td>51.2</td><td>4,263.1</td></tr><tr><td>Revisions of previous estimates</td><td>-127.2</td><td>20.7</td><td>15.0</td><td>-91.5</td></tr><tr><td>Purchases in place</td><td>41.3</td><td>—</td><td>—</td><td>41.3</td></tr><tr><td>Extensions, discoveries and other additions</td><td>951.4</td><td>—</td><td>4.6</td><td>956.0</td></tr><tr><td>Sales in place</td><td>-22.2</td><td>—</td><td>—</td><td>-22.2</td></tr><tr><td>Production</td><td>-351.2</td><td>-97.1</td><td>-11.2</td><td>-459.5</td></tr><tr><td>Net proved reserves at December 31, 2018</td><td>4,390.6</td><td>237.0</td><td>59.6</td><td>4,687.2</td></tr><tr><td>Oil Equivalents (MBoe)<sup>(2)</sup></td><td></td><td></td><td></td><td></td></tr><tr><td>Net proved reserves at December 31, 2015</td><td>2,052,361</td><td>53,843</td><td>11,913</td><td>2,118,117</td></tr><tr><td>Revisions of previous estimates</td><td>145,542</td><td>4,978</td><td>1,722</td><td>152,242</td></tr><tr><td>Purchases in place</td><td>42,330</td><td>—</td><td>—</td><td>42,330</td></tr><tr><td>Extensions, discoveries and other additions</td><td>198,973</td><td>9,990</td><td>—</td><td>208,963</td></tr><tr><td>Sales in place</td><td>-167,669</td><td>—</td><td>—</td><td>-167,669</td></tr><tr><td>Production</td><td>-183,145</td><td>-21,150</td><td>-2,755</td><td>-207,050</td></tr><tr><td>Net proved reserves at December 31, 2016</td><td>2,088,392</td><td>47,661</td><td>10,880</td><td>2,146,933</td></tr><tr><td>Revisions of previous estimates</td><td>205,262</td><td>-4,493</td><td>1,249</td><td>202,018</td></tr><tr><td>Purchases in place</td><td>2,332</td><td>—</td><td>—</td><td>2,332</td></tr><tr><td>Extensions, discoveries and other additions</td><td>385,354</td><td>29,340</td><td>6,104</td><td>420,798</td></tr><tr><td>Sales in place</td><td>-20,687</td><td>—</td><td>—</td><td>-20,687</td></tr><tr><td>Production</td><td>-203,351</td><td>-19,366</td><td>-1,707</td><td>-224,424</td></tr><tr><td>Net proved reserves at December 31, 2017</td><td>2,457,302</td><td>53,142</td><td>16,526</td><td>2,526,970</td></tr><tr><td>Revisions of previous estimates</td><td>-10,500</td><td>3,272</td><td>2,544</td><td>-4,684</td></tr><tr><td>Purchases in place</td><td>11,640</td><td>—</td><td>—</td><td>11,640</td></tr><tr><td>Extensions, discoveries and other additions</td><td>668,972</td><td>—</td><td>778</td><td>669,750</td></tr><tr><td>Sales in place</td><td>-4,509</td><td>—</td><td>-6,310</td><td>-10,819</td></tr><tr><td>Production</td><td>-245,127</td><td>-16,478</td><td>-3,406</td><td>-265,011</td></tr><tr><td>Net proved reserves at December 31, 2018</td><td>2,877,778</td><td>39,936</td><td>10,132</td><td>2,927,846</td></tr></table>

Question: What is the difference between the greatest Revisions of previous estimates in 2016 and 2015？