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I will use {ordinal} clm() (and other cool R packages such as {text2vec}

as well) here to develop a hybrid content-based, collaborative

filtering, and (obivously) model-based approach to solve the

recommendation problem on the MovieLens 100K dataset in R. All R code

used in this project can be obtained from the respective GitHub repository; the chunks of code present in the body of the post illustrate the essential steps only. The MovieLens 100K dataset can be obtained from the GroupLens research laboratory

of the Department of Computer Science and Engineering at the University

of Minnesota. The first part of the study introduces the new approach

and refers to the feature engineering steps that are performed by the OrdinalRecommenders_1.R script (found on GitHub). The second part, to be published soon, relies on the R code in OrdinalRecommenders_3.R and presents the model training, cross-validation, and analyses steps. The OrdinalRecommenders_2.R

script encompasses some tireless for-looping in R (a bad habbit indeed)

across the dataset only in order to place the information from the

dataset in the format needed for the modeling phase. The study aims at

(a) the demonstration of the improvement in predicted ratings for

recommending on a well-known dataset, and (b) attempts to shedd light on

the importance of various types of information in the work of

recommendation engines. Consequently, the code is not suited for use in

production; additional optimizations are straightforward, simple, and

necessary as well.

**Introduction**

Avoiding the formal exposition of the problem

altogether, the recommendation problem in the context of contemporary

online markets can expressed in the following way: given (A) the

information on the user’s past ratings of various items (i.e. the user-item ratings;

for example, the ratings on a 5-point Likert type scale of the movies

or TV shows that she or he has watched in the past, the ratings on

similar scales of the items purchased, and similar), (B) the information

on the user-item ratings of other users, © the attributes of users

and/or items (c.f. the user’s gender, geographical location, age,

occupation, and similar; the genre of the movie, product type, and

similar for items), provide a prediction of the user’s rating of a novel, previously unassessed item.

The predictions are then used to select the novel items (or lists of

items) that a particular user would – by assumption – enjoy to assess,

and eventualy purchase, in the future. The cognitive systems used for

making such predictions are known as recommendation engines, or recommender systems, and are widely used nowadays across the Internet business.

It is generally recognized that recommendation engines can be grouped in two broad categories: (1) content-based systems, or (2) collaborative filtering systems, with the later additionally described as (1.1) neighborhood or (1.2) model-based methods [1]. Content based

systems (CF) rely on a typical description of items over feature

vectors, and then recommend novel items to users by computing some

similarity metric between them and the items that the user has already

rated. Collaborative filtering systems, on the

other hand, rely on the assumption that the covariations between the

ratings (a) from different users over the same set of items, or (b) to

different items from the same set of users, implicitly carry the

information on item and user attributes. The information about the true

user and item attributes are thus latent in CF. In the neighborhood (or memory) variant of CF, these approaches use the user-item ratings directly and proceed by aggregating the ratings of similar users (user-based CF) and/or similar items (item-based CF)

in order to provide an estimate of the user’s rating of a novel item,

typically weighting the known ratings of similar users/items by the

respective proximity/similarity measures. In the model-based

variant of CF, the systems tries to discover the structure of the

latent factors from the observed behavioral measures (ie. known

user-item ratings), and then a myriad of machine learning algortihms and

statistical approaches can be used to train a predictive model for the

novel user-item ratings of novel items.

I will here introduce a new hybrid approach to

recommender systems with the following characteristics. First, the

system is both content-based and CF. The content-based component of the

system encompasses two matrices: the user-user and the item-item

proximity matrices, both obtained from applying the relevant distance

metric over a set of features that characterize users and items,

respectively. The CF component of the system, relies on the typical

user-user and item-item similarity matrices computed from the known,

past user-item ratings, providing for a memory component of the

recommender. Second, the system is model-based in the sense that it

combines the information from these four matrices in an ordinal logistic

regression model to predict the ratings of novel items on a 5-point

Likert scale. The underlying logic of this approach is to try to express

the recommendation problem as a discrete choice problem, where the

alternatives are provided by ordinal information from a 5-point Likert

scale, while the decision makers and the choice context are described by

a set of content-based and memory-based features, all referring to some

degree of closeness between the unknown user/item combination with the

known user/item in two spaces: the attribute (feature) space and the

neighborhood (memory) space. Because the CF approaches rely on

users-user (or item-item) proximity, the question of how many users (or

items) that are similar to the context of the present, novel user-item

rating should be used in prediction arises naturally. By expressing the

recommender problem as a regression problem essentially, we become able

to judge the significance of additional user-user and/or item-item

similarity information from the change in regression coefficients

obtained by progressively adding more and more information to nested

ordinal logistic models and training them. Thus the new approach also

provides and excellent opportunity for a comparison between

content-based information and memory-based information (the later as

being typically used in CF): which one is more important? Given the

assumption that the former is implicitly present in the later, i.e. that

it presents latent information in the CF approach, does the usage of

manifest memory-based information on user-item ratings completelly

eliminates the need for a content-based description? If not, how the two

should be combined? I will demonstrate how the present approach

resolves this dilemma while resulting in highly improved recommendations

on a well-known and widely used MovieLens dataset, at the same time

reducing drastically the number of features that are needed to build the

relevant prediction model.

**Motivation**

Consider the following user-item ratings matrix, encompassing 10 items (columns) and three users (rows):

In this rating matrix, the ratings of user1 and user2

have a maximal Pearson correlation of 1; however, the ratings of user2

and user3 have a linear correlation of .085, sharing around .007 % of

variance only. Upon a closer look, user1 and user2 have only three

common ratings (for items i1, i4, and i5), while user2 and user3 have

rated exactly the same items (i1, i3, i4, i5, i8, i9, and i10). Imagine

that the items rated by user2 and user 3 are all Sci-Fi movies; we can

imagine these two users chating and challenging each other’s views on

the contemporary Sci-Fi production on online forums or fan conferences,

right? They do not have to have perfectly similar tastes, right, but in

fact they are similar: simply because they both enjoy Sci-Fi. The Jaccard distance

measures the proximity of two vectors A and B, both given over binary

features (e.g. present vs. not present), by computing the ratio of (a)

the difference between the union of A and B and the intersection of A

and B to the (b) union of A and B; it ranges from zero to one and can be

viewed as a proportion of common features that the two vectors share in

the total number of features present in both. The Jaccard similarity

coefficient is given by one minus the Jaccard distance; we will use the

{proxy} dist() function to compute these from our ratings matrix:

### --- Illustration

user1 <- c(5, NA, NA, 4, 3, NA, 1, NA, NA, NA)

user2 <- c(4, NA, 3, 3, 2, NA, NA, 1, 2, 4)

# -- User1 and User2 have a high Pearson Correlation Coefficient:

cor(user1, user2, use = "pairwise.complete.obs")

user3 <- c(3, NA, 1, 5, 4, NA, NA, 2, 5, 4)

# -- User2 and User3 have a low Pearson Correlation Coefficient:

cor(user2, user3, use = "pairwise.complete.obs")

# -- User2 and User3 have a high Jaccard similarity

# -- i.e. 1 minus the Jaccard distance between them:

library(proxy)

as.numeric(1 - dist(t(user2), t(user3), method = "jaccard"))

as.numeric(1 - dist(t(user1), t(user2), method = "jaccard"))

as.numeric(1 - dist(t(user1), t(user3), method = "jaccard"))

User2 and User3 who have provided the ratings for the

same items exactly have a Jaccard similarity index of 1; they both have

the Jaccard similarity index of .375 with User1. Then who is the user

similar to User2 – the one whose ratings could be used to approximate

the experiences of User2 with previously unseen items: (a) User1, with

whom he has a memory-based Pearson correlation of 1 on three shared

items, or (b) User3, with whom he has a Pearson correlation of only

.085, but with whom he had shared the same experiences and thus achieved

a maximum level of Jaccard similarity?

The value of the linear correlation coefficient (or

cosine similarity, or any other similar metric) will not preserve the

information on the number of shared items; that information is imlicitly

present in the Type I Error test (for example), not in the extent of

covariance itself. The Jaccard distance, on the other hand, neglects the

covariances from the rating scales altogether, preserving only the

information about the extent of the shared ratings. If we think about

the items rated by a particular user, neglecting the particular rating

values, we understand that we can describe some aspects of the user’s

taste by binarizing his or her ratings; we can know, for example, the

frequency distribution of the item types that he or she has decided to

assess at all. If we additionally compute a distance or similarity

matrix over that information for all users, we express the content-based

approach in terms of the attributes that describe a particular use by

its distance or similarity from other users in terms of the very same

attributes – in a content-based description space. We can add the

avaialable demographic information to the binary vectors before

computing the similarities too and thus enrich the representation. And

we can do the same for items, of course. As we already have the

similarity matrices from linear correlations between the paired users’

ratings at our disposal, we can combine the information from these two

approaches – the content-based and the memory, or neighborhood-based –

and try to predict the unobserved user-item ratings from a unified

model.

The details of feature engineering along these lines in R are provided in the respective section below.

**Dataset**

The MovieLens 100K dataset [2] comprises 100,000

ratings (Likert type 1 to 5 scale) from 943 users on 1682 movies;

additionally, there are at least 20 movies rated per user. Some

demographic information for the users is present – age, gender,

occupation, zip – as well as the genre and the release dates for movies.

The dataset can be downloaded from the GroupLens research laboratory

of the Department of Computer Science and Engineering at the University

of Minnesota. Historically, it is among the most popular datasets used

to test the new approaches to the development of recommendation engines.

MovieLens 100K can be also obtained from Kaggle and Datahub. Our OrdinalRecommenders_1.R

script reads in the components of this dataset and computes the desired

user-user and item-item distance and similarity matrices following some

data reshaping.

**Feature engineering**

Part 1.A of the OrdinalRecommenders_1.R

script is used merely to read the original MovieLens 100K dataset

files, rename the columns present there, and save as .csv files; you

will need to run it too in order to use the code from part 1.B and the

remaining scripts. In Part 1.B we begin to operate over the following

three data.frames: (1) the ratingsData, which comprises the ratings on

5-point scales for all user-item pairs from the dataset, (2) usersData,

which comprises demographic information on users (Gender, Age,

Occupation, Zip-code), and (3) moviesData, which comprises all item

information (Title, Release Date, and Genre). Part 1.B of this R script

performs the essentialy feature engineering operations in order to

produce the following four matrices:

Matrix P1 – the Item-Item Proximity Matrix:

the Jaccard distances between the items, computed from the binarized

features encompassing relase decade (derived from the original release

year information, provided in the movie Title column) and movie genre.

This matrix plays the role of the content-based representation of items. The dist2() function from {text2vec} is used on a sparse Matrix class to compute the Jaccard distances here:

### --- Compute moviesDistance w. Jaccard {text2vec} from movie genres

moviesData <- moviesData %>% separate(col = Date,

into = c('Day', 'Month','Year'),

sep = "-")

moviesData$Day <- NULL

moviesData$Month <- NULL

moviesData$Year <- as.numeric(moviesData$Year)

range(moviesData$Year)

# - that would be: [1] 1922 1998

# - Introduce Movie Decade in place of Year:

decadeBoundary <- seq(1920, 2000, by = 10)

moviesData$Year <- sapply(moviesData$Year, function(x) {

wL <- x < decadeBoundary

wU <- x >= decadeBoundary

if (sum(wL) == length(decadeBoundary)) {

return(1)

} else if (sum(wU) == length(decadeBoundary)) {

decadeBoundary[length(decadeBoundary)]

} else {

decadeBoundary[max(which(wL-wU == -1))]

}

})

# - Match moviesData$Year with ratingsData:

mD <- moviesData %>%

select(MovieID, Year)

ratingsData <- merge(ratingsData, mD,

by = 'MovieID')

# - Movie Year (now Decade) as binary:

moviesData <- moviesData %>%

spread(key = Year,

value = Year,

fill = 0,

sep = "_")

# - compute moviesDistance:

moviesDistance <- moviesData[, 3:ncol(moviesData)]

w <- which(moviesDistance > 0, arr.ind = T)

moviesDistance[w] <- 1

moviesDistance <- dist2(Matrix(as.matrix(moviesData[, 4:ncol(moviesData)])),

method = "jaccard")

moviesDistance <- as.matrix(moviesDistance)

rm(moviesData); gc()

# - save objects and clear:

numMovies <- length(unique(ratingsData$MovieID))

write_csv(as.data.frame(moviesDistance),

path = paste0(getwd(),'/moviesDistance.csv'),

append = F, col_names = T)

rm(moviesDistance); gc()

Matrix P2 – the User-User Proximity Matrix:

the Jaccard distances between the users, computed from the binarized

features representing whether a particular user has or has not rated a

particular item only. This matrix plays the role of the content-based representation of users:

### --- produce binary User-Item Matrix (who rated what only):

userItemMat <- matrix(rep(0, dim(usersData)[1]*numMovies),

nrow = dim(usersData)[1],

ncol = numMovies)

userItemMat[as.matrix(ratingsData[c('UserID', 'MovieID')])] <- 1

rm('w', 'ratingsData', 'usersData'); gc()

### --- Compute userDistance w. Jaccard {text2vec}

userItemMat <- Matrix(userItemMat)

usersDistance <- dist2(userItemMat,

method = "jaccard")

rm(userItemMat); gc()

usersDistance <- as.matrix(usersDistance)

write_csv(as.data.frame(usersDistance),

path = paste0(getwd(),'/usersDistance.csv'),

append = F, col_names = T)

rm(usersDistance); gc()

Note that I’ve missed to use the user demographics

available in MovieLens 100K. In this project, I will not use them at

all, but the general ratio is: because I am going after an ordinal

logistic regression model, I can use these information as regressors

there. Also, as I have computed the user-user distance by looking at

what movies ratings they have in common, presumeably capturing some

information on shared tastes, I could have done the some for the items,

by inspecting what movies have been rated by the same users. However, a

hunch told me that I might be end up using redundand feature

representations in the same model; I’ve decided to describe the movie

similarities from content-based features directly (genre and the decade

of production), and user similarities from content-based features that

encompass the similarity in tastes (the common movie ratings among

them).

The following two similarity matrices are those

typically used in user and item-based CF; they comprise Pearson

correlation coefficients computed directly from the ratings matrix for

1681 items (note: one movie was ‘unknown’, so I have removed it from the

dataset and cleaned up from the ratings matrix accordingly) and 943

users. Given the moderate size of the desired correlation matrices, I

have used the {base} cor() function on pairwise complete observations to

compute them. For digging out correlation or cosine matrices from

problems larger than the MovieLens 100K, you might want to take a look

at the functions provided by my colleague Stefan Nikolić in his “Recommender Systems: Matrix operations for fast calculation of similarities” (supporting the computations for CF in an order of magnitude lesser time than the popular {recommenderlab} R package). The code produces the remaining two matrices:

Matrix S1 – the User-User Similarity Matrix:

the pairwise complete Pearson correlation coefficients between the

users’ ratings from the ratings matrix directly. This matrix plays the

role of the memory-based representation of users:

### --- Compute User-User and Item-Item Ratings Similarity Matrices

ratingsData <- read_csv("ratingsData_Model.csv",

col_names = T)

ratingsData$X1 <- NULL

# - User-Item Ratings Matrix

ratingsData$Timestamp <- NULL

ratingsData <- ratingsData %>%

spread(key = MovieID,

value = Rating,

sep = "_") %>%

arrange(UserID)

# - Pearson Correlations: User-User Sim Matrix

UserUserSim <- ratingsData %>%

select(starts_with("Movie"))

UserUserSim <- t(UserUserSim)

UserUserSim <- cor(UserUserSim,

use = 'pairwise.complete.obs')

UserUserSim <- as.data.frame(UserUserSim)

write_csv(UserUserSim,

path = paste0(getwd(),'/UserUserSim.csv'),

append = F, col_names = T)

rm(UserUserSim); gc()

Matrix S2 – the Item-Item Similarity Matrix:

the pairwise complete Pearson correlation coefficients between the

item’ ratings from the ratings matrix directly. This matrix plays the

role of the memory-based representation of items:

# - Pearson Correlations: Item-Item Sim Matrix

ItemItemSim <- ratingsData %>%

select(starts_with("Movie"))

rm(ratingsData); gc()

ItemItemSim <- cor(ItemItemSim,

use = 'pairwise.complete.obs')

ItemItemSim <- as.data.frame(as.matrix(ItemItemSim))

write_csv(ItemItemSim,

path = paste0(getwd(),'/ItemItemSim.csv'),

append = F, col_names = T)

rm(ItemItemSim); gc()

**Summary**

The idea is to combine the content-based with

memory-based information in a unified model to solve the recommendation

problem for online recommender systems. I have used the Jaccard

distances to describe the content-based proximity between users, based

on what items they have rated or not; on the other hand, the Jaccard

distances were also used to described the proximity of items in a

content-based space derived from the “essentialistic” item features like

movie genres and the decade of their production. Additionally, the

typical Pearson correlation similarity matrices – computed directly from

the ratings matrix – were produced for both users and items. The two

user-user and item-item matrices share the same respective

dimensionality. In the next step, the idea is to use the neighborhoods

of different size from these matrices (say, take p similar users from the content-based proximity and the memory-based user-user similarity matrices, and q

similar items from the content-based proximity and the memory-based

item-item matrices) as regressors in an ordinal logistic model to

predict the ordinal 5-point ratings of novel user-item combinations. By

inspecting the regression coefficients from these predictive models

while varying the size of the neighborhood sets of users and items whose

ratings will be used as regressors, I hope to be able to provide some

conclusion on the relative importance of content-based and memory-based

information for building efficient recommendation engines. We’ll see.

But before skipping to OrdinalRecommenders_3.R to play with the beautiful {ordinal} models, you might wish to run OrdinalRecommenders_2.R

that will prepare the first 100 neighbours from the proximity and

similarity matrices for you. As I told you, there’s a for loop there –

and a bad one.

**References**

[1] Desrosiers, C. & Karypis, G. (2010). A comprehensive survey of neighborhood-based recommendation methods. In Ricci, F., Rokach, L., Shapira, B. & Kantor, P. B. (Eds), Recommender Systems Handbook. Springer: US. DOI:10.1007/978-0-387-85820-3.

[2] F. Maxwell Harper and Joseph A. Konstan. 2015. The MovieLens Datasets: History and Context. ACM Transactions on Interactive Intelligent Systems (TiiS) 5, 4, Article 19 (December 2015), 19 pages. DOI=http://dx.doi.org/10.1145/2827872.

Goran S. Milovanović, PhD

Data Science Consultant, SmartCat

**leave a comment**for the author, please follow the link and comment on their blog:

**The Exactness of Mind**.

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