The Corporation uses observable market values for deter- mining the fair value of its trading instruments. The majority of valuations are based on actively quoted market values. In cases where actively quoted prices are not available, other external sources are used which incorporate information about commodity prices in actively quoted markets, quoted prices in less active markets and other market fundamental analysis. Internal estimates are based on internal models incorporating underlying market information such as com- modity volatilities and correlations. The Corporation’s risk management department regularly compares valuations to independent sources and models. Trading: The trading partnership in which the Corpora- tion has a 50% voting interest trades energy commodities and derivatives. The accounts of the partnership are con- solidated with those of the Corporation. The Corporation also takes trading positions for its own account. These strategies include proprietary position management and trading to enhance the potential return on assets. The infor- mation that follows represents 100% of the trading partner- ship and the Corporation’s proprietary trading accounts. In trading activities, the Corporation is exposed to changes in crude oil, natural gas and refined product prices, primarily in North America and Europe. Trading positions include futures, swaps and options. In some cases, physical pur- chase and sale contracts are used as trading instruments and are included in the trading results. Gains or losses from sales of physical products are recorded at the time of sale. Derivative trading transactions are marked-to-market and are reflected in income currently. Total realized gains for the year amounted to $50 million. The following table provides an assessment of the factors affecting the changes in fair value of trading activities and represents 100% of the trading partnership and other trading activities. The following table summarizes the value-at-risk results for all trading activities. The results may change from time to time as strategies change to capture potential market rate movements. Millions of dollars Total 2004 2005 2006 Source of fair value Prices actively quoted $69 $33 $36 $GLYPH<209> Other external sources 5 (8) 7 6 Internal estimates (7) (4) (3) GLYPH<209> Total $67 $21 $40 $ 6 Trading Millions of dollars Activities 2003 At December 31 $7 Average for the year 9 High during the year 12 Low during the year 7 2002 At December 31 $6 Average for the year 10 High during the year 12 Low during the year 6 Millions of dollars 2003 2002 Fair value of contracts outstanding at  the beginning of the year $36 $(58) Change in fair value of contracts  outstanding at the beginning of  the year and still outstanding at the  end of year 36 (14) Reversal of fair value for contracts closed  during the year (26) 75 Fair value of contracts entered into  during the year and still outstanding 21 33 Fair value of contracts outstanding  at the end of the year $67 $36 27