The proven Zacks Rank system focuses on earnings estimates and estimate revisions to find winning stocks. Nevertheless, we know that our readers all have their own perspectives, so we are always looking at the latest trends in value, growth, and momentum to find strong picks.Looking at the history of these trends, perhaps none is more beloved than value investing. This strategy simply looks to identify companies that are being undervalued by the broader market. Value investors use fundamental analysis and traditional valuation metrics to find stocks that they believe are being undervalued by the market at large.Luckily, Zacks has developed its own Style Scores system in an effort to find stocks with specific traits. Value investors will be interested in the system's "Value" category. Stocks with both "A" grades in the Value category and high Zacks Ranks are among the strongest value stocks on the market right now.One stock to keep an eye on is Marathon Oil (MRO). MRO is currently sporting a Zacks Rank of #2 (Buy), as well as an A grade for Value.We also note that MRO holds a PEG ratio of 0.47. This metric is used similarly to the famous P/E ratio, but the PEG ratio also takes into account the stock's expected earnings growth rate. MRO's PEG compares to its industry's average PEG of 0.94. Over the last 12 months, MRO's PEG has been as high as 0.59 and as low as -1.69, with a median of -1.01.Another notable valuation metric for MRO is its P/B ratio of 0.90. The P/B is a method of comparing a stock's market value to its book value, which is defined as total assets minus total liabilities. This stock's P/B looks attractive against its industry's average P/B of 1.47. Over the past year, MRO's P/B has been as high as 1.05 and as low as 0.28, with a median of 0.56.Finally, our model also underscores that MRO has a P/CF ratio of 11.23. This metric focuses on a firm's operating cash flow and is often used to find stocks that are undervalued based on the strength of their cash outlook. This stock's P/CF looks attractive against its industry's average P/CF of 25.44. Over the past 52 weeks, MRO's P/CF has been as high as 13 and as low as 1.79, with a median of 6.89.These are only a few of the key metrics included in Marathon Oil's strong Value grade, but they help show that the stock is likely undervalued right now. When factoring in the strength of its earnings outlook, MRO looks like an impressive value stock at the moment. 5 Stocks Set to Double Each was hand-picked by a Zacks expert as the #1 favorite stock to gain +100% or more in 2021. Each comes from a different sector and has unique qualities and catalysts that could fuel exceptional growth. Most of the stocks in this report are flying under Wall Street radar, which provides a great opportunity to get in on the ground floor. Today, See These 5 Potential Home Runs >>Want the latest recommendations from Zacks Investment Research? Today, you can download 7 Best Stocks for the Next 30 Days. Click to get this free report Marathon Oil Corporation (MRO): Free Stock Analysis Report To read this article on Zacks.com click here. Zacks Investment Research