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, 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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