I am trying to use Spark MLib ALS with implicit feedback for collaborative filtering. Input data has only two fields userId and productId. I have no product ratings, just info on what products users have bought, that's all. So to train ALS I use:

def trainImplicit(ratings: RDD[Rating], rank: Int, iterations: Int): MatrixFactorizationModel


This API requires Rating object:

Rating(user: Int, product: Int, rating: Double)

On the other hand documentation on trainImplicit tells: Train a matrix factorization model given an RDD of 'implicit preferences' ratings given by users to some products, in the form of (userID, productID, preference) pairs.

When I set rating / preferences to 1 as in:

val ratings = sc.textFile(new File(dir, file).toString).map { line =>
  val fields = line.split(",")
  // format: (randomNumber, Rating(userId, productId, rating))
  (rnd.nextInt(100), Rating(fields(0).toInt, fields(1).toInt, 1.0))

 val training = ratings.filter(x => x._1 < 60)
val validation = ratings.filter(x => x._1 >= 60 && x._1 < 80)
val test = ratings.filter(x => x._1 >= 80).values.cache()

And then train ALSL:

 val model = ALS.trainImplicit(ratings, rank, numIter)

I get RMSE 0.9, which is a big error in case of preferences taking 0 or 1 value:

val validationRmse = computeRmse(model, validation, numValidation)

/** Compute RMSE (Root Mean Squared Error). */
 def computeRmse(model: MatrixFactorizationModel, data: RDD[Rating], n: Long): Double = {
val predictions: RDD[Rating] = model.predict(data.map(x => (x.user, x.product)))
val predictionsAndRatings = predictions.map(x => ((x.user, x.product), x.rating))
  .join(data.map(x => ((x.user, x.product), x.rating)))
math.sqrt(predictionsAndRatings.map(x => (x._1 - x._2) * (x._1 - x._2)).reduce(_ + _) / n)

So my question is: to what value should I set rating in:

Rating(user: Int, product: Int, rating: Double)

for implicit training (in ALS.trainImplicit method) ?



  val alpha = 40
  val lambda = 0.01

I get:

Got 1895593 ratings from 17471 users on 462685 products.
Training: 1136079, validation: 380495, test: 379019
RMSE (validation) = 0.7537217888106758 for the model trained with rank = 8 and numIter = 10.
RMSE (validation) = 0.7489005441881798 for the model trained with rank = 8 and numIter = 20.
RMSE (validation) = 0.7387672873747732 for the model trained with rank = 12 and numIter = 10.
RMSE (validation) = 0.7310003522283959 for the model trained with rank = 12 and numIter = 20.
The best model was trained with rank = 12, and numIter = 20, and its RMSE on the test set is 0.7302343904091481.
baselineRmse: 0.0 testRmse: 0.7302343904091481
The best model improves the baseline by -Infinity%.

Which is still a big error, I guess. Also I get strange baseline improvement where baseline model is simply mean (1).

  • 2
    $\begingroup$ I was under the impression that RMSE only worked for explicit ratings. $\endgroup$
    – dudemonkey
    Nov 21, 2016 at 14:41

1 Answer 1


Lets start with an example: Say you have data on transaction details of customers in a store. So you have who bought what and when. Clearly you don't have rating data, but you do have the understanding that if a person buys an item multiple times, maybe he likes it(Or would rate it high if given the chance). Thus an implicit preference/Rating could be "How many times someone buys something".

You can also combine the time component, e.g."A" buys chips 5 times this week, but "B" bought it 5 times the last week. If your preference is "#purchases in last month", you'd miss out the information that maybe "B" has lesser chances of buying chips again than "A". So you can add a time-decay to aggregate the counts.

In the RDD<Rating>, you can provide these values directly, what trainImplicit() would do is train a value between 0 and 1 (p) which depends on how high the implicit preference is. Now when you use the model to predict, you should only expect value in range 0 to 1, and not the count that you provided in RDD<Rating>.

  • $\begingroup$ What if a small fraction of the users buy just one at most two different items and others buy nothing at all? What to recommend to these others? $\endgroup$
    – zork
    Feb 9, 2016 at 18:31
  • $\begingroup$ For users with only one item,you can still recommend using the same ALS model. For those who don't have anything, what amazon does is recommending global top picks, which can be further refined by using user demographic and then selecting the top picks for that demographic $\endgroup$ Feb 11, 2016 at 1:38

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