However, a big fall in a companys share price doesnt always make it cheap.
Whats more, although these falls have been painful for shareholders, they are really just short-term fluctuations in the valuation of long-term businesses.
As investors, we need a more reliable way to identify genuinely cheap buying opportunities.
One such method is the PE10 ratio, which divides the current share price by a firms ten-year average earnings per share.
The result shows how the company is valued against its earning power over a significant period of time. Thats very useful, especially with oil and gas companies, whose profits can be very volatile from year to year, as the price of oil and gas fluctuates.
BP vs BG
Ive calculated a PE10 value for BP and BG the results may surprise you:
Source: company reports
BG Group is valued at almost 15 times its ten-year average earnings, while BP trades on just seven times ten-year average earnings per share.
A growth investor might say that this is because BP is shrinking, having sold nearly $50bn of assets to fund the costs of the Gulf of Mexico disaster, while BG Group is a growth story thats about to start generating lots of cash.
Perhaps, but Im not convinced and heres why.
Theres already a lot of bad news priced into BP shares. Take your pick BP is expected to report a loss from its 19.75% stake in Rosneft, the firms generous dividend might be cut, and it could face a $20bn fine in the US this year.
Good news is thin on the ground for BP shareholders.
At BG, on the other hand, the market is still quite optimistic. BG shares currently trade on 15 times 2015 forecast earnings compared to 10 for BP.
BG recently reported the first shipment of gas from its Australian QCLNG project, and its secured the services of Statoils highly-rated chief executive, Helge Lund, who will start work at BG in March.
Yet BG isnt without problems: its got too much debt, and has also been hit by the falling price of oil.
Whats more, after a turbulent few years, it would be surprising if Mr Lund didnt find a few more skeletons in the firms cupboards when he takes control, which could lead to a painful kitchen sink update soon after he starts work.
As a result of all this, I believe BP is a buy, but BG Group remains a sell.
Of course, you may not agree. But whether you share my view or not, I’d urge you to consider the five stage dividend test in “How To Create Dividends For Life“.
Applying these rules to your dividend stock picks could help you avoid painful losses and outperform the wider market.
The report is easy to understand, FREE and without obligation.
To receive your copy immediately, click here now.
Get FREE Issues of The Motley Fool Collective
Get straightforward advice on whats really happening with the stock markets, direct to your inbox. Help yourself with our FREE email newsletter designed to help you protect and grow your portfolio wealth.