Javascript false positive and negative rate

gistfile1.js

var user  = [0,1,0,0,1,0,0,1,1,0,0,0,1,0],
    truth = [0,1,1,1,0,0,0,0,1,1,0,0,1,1];

function compare(user, truth) {
  var a = 0, b = 0, c = 0, d = 0, fpr, fnr;
  if( truth.length !== user.length ) return;
  for (var i = 0; i < truth.length; i++) {
    switch(user[i] + truth[i]) {
      case 0: ++d; break;
      case 2: ++a; break;
      case 1: user[i] ? ++c : ++b; break;
    }
  }
  if( a+b+c+d !== truth.length ) return;
  fpr = c/(a+c); fnr = b/(b+d);
  return {'true_pos': a, 'false_neg': b, 'false_pos': c, 'true_neg': d, 'false_pos_rate': fpr, 'false_neg_rate': fnr}
}