public Pearson(UserPreferences user1, UserPreferences user2) {
		userId1 = user1.getUserId();
		userId2 = user2.getUserId();
		intersection(user1, user2);
		if (itemIds.length == user1.getItemIds().length && itemIds.length == user2.getItemIds().length) {
			return;
		}
		pearson = calculatePearson();
		calculateWeights();
		
	}