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The poll average generally uses the last 5 polls (by middate).
If there is tie for the middate of the oldest poll to be included, all polls with that middate are included.
If the result is exactly on the border between categories (0%, 5% or 10% margin) older polls are pulled in one by one until the result is clearly within a category.
When there are not enough actual polls for the poll average, results from prior presidential elections are used to fill in the average.
If a pollster releases multiple results based on the same sample they are weighted so collectively they count as "1 poll".
On state detail pages this is noted by an [N], shading in the listing, and a different color data point in the graph.
State win probabilities are calculated based on 2008-2016 data using the methodology in the January 2019 blog post titled "Polling Error vs Final Margin".
Analysis for the 2008 and 2012 election cycles can be found here.