org.grouplens.lenskit.transform.normalize

Class MeanVarianceNormalizer

java.lang.Object

org.grouplens.lenskit.transform.normalize.AbstractVectorNormalizer

org.grouplens.lenskit.transform.normalize.MeanVarianceNormalizer

All Implemented Interfaces:

Serializable, VectorNormalizer

@DefaultProvider(value=MeanVarianceNormalizer.Builder.class)
 @Shareable
public class MeanVarianceNormalizer
extends AbstractVectorNormalizer
implements Serializable

Normalizes against the variance of the vector with optional smoothing as described in Hofmann '04.

For user rating vectors, this normalization assumes that a user's mean rating and variance are independent of actual preferences, and attempts to describe the preference of a rating by the distance of the rating from the mean, relative to the user's normal rating variance.

The smoothing factor helps to smooth out results for users with fewer ratings by re-weighting the user's rating variance. The 'smoothing number' is a number of 'default' ratings to give the user, weighting the user's variance towards the average community variance. Accordingly, set smoothing = 0 (or use default constructor) for no smoothing. The 'global variance' parameter only pertains to smoothing, and is unnecessary otherwise.

See Also:

Serialized Form

Nested Class Summary

Nested Classes

Modifier and Type	Class and Description
static class	MeanVarianceNormalizer.Builder A Builder for UserVarianceNormalizers that computes the variance from a RatingBuildContext.

Constructor Summary

Constructors

Constructor and Description
MeanVarianceNormalizer() Initializes basic normalizer with no damping.
MeanVarianceNormalizer(double damping, double globalVariance) Construct a new mean variance normalizer.

Method Summary

Modifier and Type	Method and Description
double	getDamping() Get the damping term.
double	getGlobalVariance() Get the global variance.
VectorTransformation	makeTransformation(SparseVector reference) Create a vector transformation that normalizes and denormalizes vectors with respect to the specified entity.

Methods inherited from class org.grouplens.lenskit.transform.normalize.AbstractVectorNormalizer

normalize

Methods inherited from class java.lang.Object

clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait

Constructor Detail

MeanVarianceNormalizer

public MeanVarianceNormalizer()

Initializes basic normalizer with no damping.

MeanVarianceNormalizer

public MeanVarianceNormalizer(double damping,
                              double globalVariance)

Construct a new mean variance normalizer.

Parameters:

damping - damping factor to use. 0 for no damping, 5 for Hofmann's implementation.

globalVariance - global variance to use in the damping calculations.

Method Detail

getDamping

public double getDamping()

Get the damping term.

Returns:

The damping term.

getGlobalVariance

public double getGlobalVariance()

Get the global variance.

Returns:

The global variance from build time.

makeTransformation

public VectorTransformation makeTransformation(SparseVector reference)

Description copied from interface: VectorNormalizer

Create a vector transformation that normalizes and denormalizes vectors with respect to the specified entity. This allows transformations to be applied multiple times to different vectors and also unapplied.

If the reference vector is empty, the returned transformation should be the identity transform. Results are undefined if the reference vector is not complete or contains NaN values.

If the normalization needs to retain a copy of the sparse vector, it will take an immutable copy.

Specified by:

makeTransformation in interface VectorNormalizer

Parameters:

reference - The reference vector.

Returns:

A transformation built from the reference vector.