In the ordinary course of business Shell enters into contracts to supply or purchase oil and gas products, and also enters into derivative contracts to mitigate resulting economic exposures (generally price exposure). Derivative contracts are carried at period-end market price (fair value), with movements in fair value recognised in income for the period. Supply and purchase contracts entered into for operational purposes are, by contrast, recognised when the transaction occurs (see also below); furthermore, inventory is carried at historical cost or net realisable value, whichever is lower.As a consequence, accounting mismatches occur because: (a) the supply or purchase transaction is recognised in a different period; or (b) the inventory is measured on a different basis. In addition, certain UK gas contracts held by Upstream are, due to pricing or delivery conditions, deemed to contain embedded derivatives or written options and are also required to be carried at fair value even though they are entered into for operational purposes. The accounting impacts of the aforementioned are reported as identified items in the quarterly results. 6. Return on average capital employed Return on average capital employed measures the efficiency of Shell's utilisation of the capital that it employs and is a common measure of business performance. In this calculation, return on average capital employed is defined as the sum of income for the current and previous three quarters adjusted for after-tax interest expense as a percentage of the average capital employed for the same period. Capital employed consists of total equity, current debt and non-current debt. The tax rate is derived from calculations at the published segment level. LIQUIDITY AND CAPITAL RESOURCES Third quarter Net cash from operating activities in the third quarter 2012 was $9.5 billion compared with $11.6 billion for the same period last year. Total current and non-current debt increased to $36.4 billion at September 30, 2012 from $33.0 billion at June 30, 2012 while cash and cash equivalents increased to $18.8 billion at September 30, 2012, from $17.3 billion at June 30, 2012. During the third quarter 2012 Shell issued $2.5 billion of debt under the US universal shelf registration. No new debt was issued under the euro medium-term note programme. Net capital investment in the third quarter 2012 was $8.0 billion, of which $6.9 billion was in Upstream and $1.1 billion in Downstream. Net capital investment in the same period of 2011 was $6.1 billion, of which $5.9 billion was in Upstream and $0.2 billion in Downstream. Dividends of $0.43 per share are announced on November 1, 2012 in respect of the third quarter. These dividends are payable on December 20, 2012. In the case of the Class B shares, the dividends will be payable through the dividend access mechanism and are expected to be treated as UK-source rather than Dutch-source. See the Annual Report and Form 20-F for the year ended December 31, 2011 for additional information on the dividend access mechanism. Shell provides shareholders with a choice to receive dividends in cash or in shares via a Scrip Dividend Programme. Under the Scrip Dividend Programme shareholders can increase their shareholding in Shell by choosing to receive new shares instead of cash dividends. Only new Class A shares will be issued under the Programme, including to shareholders who currently hold Class B shares.