MarkWest Energy Partners, Suburban Propane Partners, and NuStar Energy all have credit ratings from Standard and Poor's that are just shy of investment grade. In this video, Fool.com contributor Aimee Duffy examines what a gathering and processing MLP like MarkWest Energy and a propane MLP like Suburban Propane must do to achieve investment grade status, and why a business like NuStar Energy might mislead investors too focused on ratings.

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Fool contributor Aimee Duffy has no position in any stocks mentioned. Fool contributor Tyler Crowe has no position in any stocks mentioned. The Motley Fool has no position in any of the stocks mentioned. Try any of our Foolish newsletter services free for 30 days. We Fools may not all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors. The Motley Fool has a disclosure policy.