Parties can play a variety of roles within the political system. They represent social groups, manage political conflict, recruit political leaders, socialize political activists and mobilize public opinion. The precise functions they fulfil clearly differ between political systems and at different periods within the same system. Duverger's work on political parties, for example, which probably did most to reawaken academic interest in the area, does not deal with the policy-making process in parties. It had two major concerns. The leading work on British political parties by Robert McKenzie, which is more like a case study, is rather different in style, but this too does not directly deal with problems of policy-making. The Labour Party is seen both as a policy-maker with the power to implement policies and as a pressure group with power only to influence.