LONDON -- Right now I'm trawling through the FTSE 100 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 Marks & Spencer (LSE: MKS.L) to see if its reported earnings have been distorted significantly by exceptional, one-off, or unusual items. I've extracted the following statistics courtesy of S&P Capital IQ:

Year to March

2008

2009

2010

2011

2012

Profit before unusual items (million pounds)

1,007

604

708

714

706

Restructuring charges (million pounds)

-

(136)

-

(16)

(18)

Goodwill impairment

-

-

-

-

(34)

Gain on sales of assets and investments (million pounds)

27

6

8

57

16

Asset writedowns

-

-

-

(6)

(10)

Other unusual items (million pounds)

95

231

(14)

31

-

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

So between 2008 and 2012, my stats tell me Marks & Spencer reported cumulative profits before exceptional items and tax of 3,739 million pounds. However, aggregate exceptional costs came to 237 million pounds -- equivalent to just 6% of cumulative "underlying" profits.

Looking at our summary table, Marks & Spencer, which reports its half-year results tomorrow, seems to have a fairly clean profit and loss account. Most of its one-off items were quite small in relation to total profits.

The main exceptions were a fairly hefty restructuring charge back in 2009, which was mostly property-related. Marks & Spencer also took two sizable credits to its profit-and-loss account in 2008 and 2009, both in relation to its company pension scheme, where the future benefits payable were made less generous.

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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