How are the top 10 free to play steam lists calculated?

In the beginning of every month we publish the top 10 most popular free games on steam of the last month. How are these monthly Steam statistics calculated?

Once per hour we gather the current amount of players of all free steam games via the Steam Web API and save them on our database. Using these measurements we calculate the average player numbers per day and per month by adding all measured players and dividing them by the number of measurements taken.