Search Volume reported for a target keyword in Keywords Explorer is different from "Traffic potential" reported for the Parent topic keyword. 

The search volume that you see shows how often a target keyword is searched per month in a given country (average for last 12 months).

We calculate this metric with exceptional accuracy by processing large amounts of clickstream data (more details here).

However, Traffic potential at the top of the report shows how much organic search traffic you can possibly get if you rank #1 for the Parent topic keyword

We estimate this Traffic potential by looking at the organic search traffic of the current #1 ranking result for that Parent topic. (For organic search traffic, we calculate it based on the keywords we see that website or URL is ranking for and their positions in the SERPs.)

Thus, Traffic potential is simply the organic search traffic of the current #1 ranking result for that Parent topic keyword.  

We do not report Traffic potential for each keyword targeted in Keywords Explorer - only for the Parent topic one, but if you wanted the same thing for other keywords you are searching, that would be the search traffic of that particular keyword. 

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