Wordcount can be challenging to calculate because there are often very different types of pages in the SERP results and so using things like averages tend to give you bad guages of the true wordcount that is required your page type.
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The best solution is to turn off any search results that are not similar to the page you will be creating. You can do that by clicking settings.
Automatic Settings
Alternatively, you can rely on our automated guesses of word count. We calculate word count by first removing any 0 values, then finding the mean and standard deviation of the collection of wordcounts. Once we have these values, we eliminate any values outside of 1.25 standard deviations of the data set.
What is left is a range of values that best capture the word counts required to rank on page 1.