normalize {recommenderlab}

Normalize the ratings

Description

Provides the generic for normalize/denormalize and a method to normalize/denormalize the ratings in a realRatingMatrix.

Usage

normalize(x, ...)
## S4 method for signature 'realRatingMatrix'
normalize(x, method="center", row=TRUE)

denormalize(x, ...)
## S4 method for signature 'realRatingMatrix'
denormalize(x, method=NULL, row=NULL, 
    factors=NULL)

Arguments

x

a realRatingMatrix.

method

normalization method. Currently "center" or "Z-score".

row

logical; normalize rows (or the columns)?

factors

a list with the factors to be used for denormalizing (elements are "mean" and "sds"). Usually these are not specified and the values stored in x are used.

...

further arguments (currently unused).

Details

Normalization tries to reduce the individual rating bias by row centering the data, i.e., by subtracting from each available rating the mean of the ratings of that user (row). Z-score in addition divides by the standard deviation of the row/column. Normalization can also be done on columns.

Denormalization reverses normalization. It uses the normalization information stored in x unless the user specifies method, row and factors.

Value

A normalized realRatingMatrix.

Examples

## create a matrix with ratings
m <- matrix(sample(c(NA,0:5),50, replace=TRUE, prob=c(.5,rep(.5/6,6))), 
nrow=5, ncol=10, dimnames = list(users=paste('u', 1:5, sep=''),
items=paste('i', 1:10, sep='')))

## do normalization
r <- as(m, "realRatingMatrix")
r_n1 <- normalize(r) 
r_n2 <- normalize(r, method="Z-score")

r
r_n1
r_n2

## show normalized data
image(r, main="Raw Data")
image(r_n1, main="Centered")
image(r_n2, main="Z-Score Normalization")