Passenger numbers entering London Underground stations, largely based on the Underground ticketing system gate data for five weeks period average, in September -November 2012.
data source: Transport For London
Z-score explained : “A Z-Score is a statistical measurement of a score’s relationship to the mean in a group of scores. A Z-score of 0 means the score is the same as the mean. A Z-score can also be positive or negative, indicating whether it is above or below the mean and by how many standard deviations. ” from Investopedia