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Hi Stephanie,

I'd say pulling your analytics data for keywords you're getting long tail traffic for that you're ranking for on the first page (not in top 2 spots) and prioritizing that against your rankings data (whether its from SEOMoz, Advanced Web Ranking etc) using Vlookups in Excel

If you're estimating potential traffic, you can export Adwords keyword data for potential traffic (very rough estimate) to get an idea of what kind of traffic you could potentially be getting

An excellent resource would be http://www.distilled.net/excel-for-seo/#vlookup to start off with.

Hope that helps!

Much like what Irving has already mentioned, rankings do fluctuate and aren't static.  Without being provided any more detail on your end - have you made any changes to your website recently on the back end?  That way we can better help you.

If it's rankings that have dropped one spot then that shouldn't be cause for alarm.