The size of the EDL movement has been difficult to gauge. Allen noted that within a year and a half of the group's formation it had "grown substantially" but that it is "extremely difficult to know exactly how big or how well supported the EDL is." The organisation has no official system of membership, or formal means of joining, and thus no membership list. In 2011, Bartlett and Littler estimated that between 25,000 and 35,000 people were active members in the EDL movement. They believed that about half of these had attended demonstrations and that the highest concentration was in the Greater London area. On the basis of her research with the group, Pilkington suggested that there was a "high turnover in the movement", while Winlow, Hall, and Treadwell observed that members "drift in and