That's why PA and DA metrics often vary from tool to tool. Each ad hoc keyword tool we tested came up with slightly different numbers based on what they're pulling from Google and other sources, and how they're doing the calculating. The shortcoming of PA and DA is that, even though they give you a sense of how authoritative a page might be in the eyes of Google, they don't tell you how easy or difficult it will be to position it for a particular keyword. This difficulty is why a third, newer metric is beginning to emerge among the self-service SEO players: difficulty scores.
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