In each year since the inception of the modern primary system, each party vote in each state (when not faced with an incumbent or other candidate with no real opposition) has resulted in a 50% success rate in choosing the party's eventual candidate: Iowa Democrats picked Carter in '76, Mondale in '84 and Kerry in '04, but they also chose Muskie in '72, Gephardt in '88 and Harkin in '92. Iowa Republicans chose Dole in '96 and Bush in '00, but they also picked Bush in '80 (over Reagan) and Dole in '88 (over Bush). New Hampshire fares no better despite their longer history: their Democratic voters picked right in '60, '76, '88, '00 and '04 (Kennedy, Carter, Dukakis, Gore and Kerry), but picked wrong in '52, '56, '72, '84 and '92 (Kefauver twice, Muskie, Hart and Tsongas). Their G.O.P. chose correctly in '52, '68, '80 and '88 (Eisenhower, Nixon, Reagan and Bush) but chose wrongly in '48, '64, '96, and '00 (Stassen, Lodge, Buchanan & McCain).
fengi has already done a fine job in explaining why the primary system, like so much of our American democracy, is unfair, distorted and undemocratic, but you'd think that at least its unfairness would get results. But no: it's a completely fucked system, and it only works half the time for all that. God bless Vespucciland.