Please take this question in the spirit of inquiry that I intend, not as pushing any particular social agenda.

Various countries and institutions have implemented forms of 'positive discrimination' in order to level the playing field between disadvantaged groups and society at large. Affirmative action is the term I have heard most commonly used in America. How effective are these programs at reducing the, typically socioeconomic, divide for a particular group as a whole? Do they change perceptions and reduce prejudice among those not part of the group? Are they effective only among the individuals who directly benefit? Are other approaches more effective, or less, or is a systemic approach including positive discrimination the most effective strategy to reducing inequality? I suppose 'effective' might be considered in terms of both reduction of division between relevant measured factors, and also the speed of that reduction, as well as reduction of negative perception or active prejudice among non-members of the particular population.