long totalEntries = 0;
for (int user = 0; user < USERS; user++) {
String userID = "U" + user;
model.setUserVector(userID, randomVector(random));
int itemsPerUser = itemPerUserDist.sample();
totalEntries += itemsPerUser;
Collection<String> knownIDs = new ArrayList<>(itemsPerUser);
for (int i = 0; i < itemsPerUser; i++) {
knownIDs.add("I" + random.nextInt(ITEMS));
}