First, the mass public is greatly influenced by a monopolistic media while the importance of separate discussion circles is diminished proportionately with the increase of media. Second, the process of forming opinion is hold and spread from a center, making media markets "huge and centralized" (idem). Third, opinion is determined by unnatural causes due to manipulation of the public who is passively receiving information and not invited to discussion. Fourth, that only in the case of authoritarian forms of government, decision making is enforced by the power of fear and violence. (idem) Mills briefly describes a survey destined to gather data from people in the American Midwest, in a small city in Illinois. The questions of the survey, conducted twice at an interval of two months, shed some light into the way people change their mind. Mass media is responsible for a big part of people's opinions, but "person-to-person discussions" proved...

Although, the "discussion phase"(Mills, p.584) seemed to be no longer existent since the public reacted as a consequence of a "stimulus presented by centralized management"(Mills, p. 584), people did not entirely loose their willingness to exchange opinions and thus the process of opinion forming was still a thing that was not as predictable as some scholars might think. The supreme example in this respect is for Mills the result of the elections from 1948, when Truman, the representative of the Democratic party was elected "against" a republican press (Mills, p10).
Resources:

Arendt, H. Communicative Power. Readings in Social and Political Theory.New York University Press.

Arendt, H.(1958) the Human Condition. The University of Chicago Press. Chicago.

Mills, W. Power, Politics and People. Oxford University Press.