This page explains how we sourced the data for the Interactive Premier League Trending Map
The data that we used was provided by Google to determine which Premier League clubs were most popular in each different country. Google Trends gives us a score out of 100, with 100 being the highest trending data point.
Google Trends only allows a maximum of 5 comparisons, so in order to get the full compliment of 20 teams, we had to first determine the team with the largest trend (their score is 100, when compared with every team, for each year, for each country). Once we found that team we used them in each request to Google Trends with 4 other teams. Knowing that we had already found the highest trending team (per year and per country) we could be confident that all other teams were percentage of that 100 score.
There are some limitations with the data, for example you cannot directly compare between countries due to how the data is focused on an individual country.
Google themselves use a sample of all searches, and is normalised themselves. Some key points to take into consideration about their normalisation process.
More information on Google's Trending Data can be found here