LONDON -- Right now I'm trawling through the FTSE 100 (UKX) and double-checking for blue chips that may be flattering their profits.

You see, many companies these days report "underlying" earnings, which are calculated by excluding costs the firm deems to be "exceptional." Trouble is, some companies are more cavalier than others when it comes to sweeping awkward expenses away from the headline figures.

Today I'm looking at BP (LSE: BP.L) (NYSE: BP) to see if its reported earnings have been distorted significantly by exceptional, one-off, or unusual items. I've extracted the following statistics:

Year to Dec. 31

2007

2008

2009

2010

2011

Profit before unusual items (£m)

15,499

23,791

15,652

(6,096)

23,626

Gain on sale of assets (£m)

1,251

929

1,345

4,088

2,653

Asset writedowns (£m)

(845)

(1,189)

(1,444)

(1,082)

(1,322)

Source: S&P Capital IQ

While annual figures can provide some insight into how a business has performed, I reckon that looking back over several years provides a better view of possible problems in relation to one-off costs.

So between 2007 and 2011, my stats tell me BP reported cumulative profits before exceptional items and tax of £72.5 billion. However, aggregate exceptional costs came to £4.4 billion -- equivalent to 6% of cumulative "underlying" profits.

The main fluctuation in BP's profits over the last five years was the Gulf of Mexico disaster, which was mostly accounted for in 2010, when the oil giant recorded a £6.1 billion loss. Apart from that, it's pleasing to see that the exceptional items moving through BP's profit and loss account have been relatively small in relation to its overall profits.

As with most big oil companies, BP spends a fair amount of its time buying and selling assets around the globe. So it's not that surprising to see regular gains on the sale of assets, combined with regular asset writedowns. What's more, the fact that gains on the sale of assets have been made in each of the last five years should give BP's investors some comfort that its accounting is relatively conservative.

Somebody who always studies earnings numbers in detail is Neil Woodford, the U.K.'s leading equity income fund manager. Woodford's portfolios thrashed the FTSE 100 during the 15 years to 2011 and this exclusive Motley Fool report -- which can be downloaded free today -- reviews his favorite blue-chip shares for 2013 and beyond.

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