# Encoding your categorical variables based on the response variable and correlations

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Sometimes in Statistical/Machine Learning problems, we encounter **categorical explanatory variables with high cardinality**. Let’s say for example that we want to determine

**if a diet is**. In trying to answer this question, we’d construct a response variable containing a sequence of characters

*good*or*bad*, based on what a person eats*good*or

*bad*, one for each person; and an explanatory variable for the model would be:

x = c("apple", "tomato", "banana", "apple", "pineapple", "bic mac", "banana", "bic mac", "quinoa sans gluten", "pineapple", "avocado", "avocado", "avocado", "avocado!", ...)

Some Statistical/Machine learning models only accept numerical data as input. Hence the need for a way to transform those categorical inputs into numerical vectors. One way to deal with a covariate such as `x`

is to use **one-hot encoding**, as depicted below:

In the case of `x`

having 100 types of fruits in it, one-hot encoding will lead to 99 explanatory variables for the model, instead of, possibly one. This means: **more disk space required, more computer memory needed, and a longer training time**. Apart from the one-hot encoder, there are a lot of categorical encoders out there. I **wanted a relatively simple one**, so I came up with the one described in this post. It’s a target-based categorical encoder, which makes use of the correlation between a randomly generated pseudo-target and the real target (a.k.a response; a sequence *good* or *bads* as seen before).

## Data and packages for the demo

We’ll be using the `CO2`

dataset available in base R for this demo. According to its description: the CO2 data frame has 84 rows and 5 columns of data from an experiment on the cold tolerance of the grass species **Echinochloa crus-gall**.

# Packages required library(randomForest) # Dataset Xy <- datasets::CO2 Xy$uptake <- scale(Xy$uptake) # centering and scaling the response print(dim(Xy)) print(head(Xy)) print(tail(Xy))

Now we create a **response variables and covariates**, based on `CO2`

data:

y <- Xy$uptake X <- Xy[, c("Plant", "Type", "Treatment" ,"conc")]

## First encoder: “One-hot”

Using base R’s function `model.matrix`

, we transform the categorical variables from `CO2`

to numerical variables. It’s not exactly “One-hot” as we described it previously, but a close cousin, because the covariate `Plant`

possesses some sort of *ordering* (it’s “an ordered factor with levels Qn1 < Qn2 < Qn3 < … < Mc1 giving a unique identifier for each plant”):

X_onehot <- model.matrix(uptake ~ ., data=CO2)[,-1] print(dim(X_onehot)) print(head(X_onehot)) print(tail(X_onehot)) ## [1] 84 14 ## Plant.L Plant.Q Plant.C Plant^4 Plant^5 Plant^6 ## 1 -0.4599331 0.5018282 -0.4599331 0.3687669 -0.2616083 0.1641974 ## 2 -0.4599331 0.5018282 -0.4599331 0.3687669 -0.2616083 0.1641974 ## 3 -0.4599331 0.5018282 -0.4599331 0.3687669 -0.2616083 0.1641974 ## 4 -0.4599331 0.5018282 -0.4599331 0.3687669 -0.2616083 0.1641974 ## 5 -0.4599331 0.5018282 -0.4599331 0.3687669 -0.2616083 0.1641974 ## 6 -0.4599331 0.5018282 -0.4599331 0.3687669 -0.2616083 0.1641974 ## Plant^7 Plant^8 Plant^9 Plant^10 Plant^11 ## 1 -0.09047913 0.04307668 -0.01721256 0.005456097 -0.001190618 ## 2 -0.09047913 0.04307668 -0.01721256 0.005456097 -0.001190618 ## 3 -0.09047913 0.04307668 -0.01721256 0.005456097 -0.001190618 ## 4 -0.09047913 0.04307668 -0.01721256 0.005456097 -0.001190618 ## 5 -0.09047913 0.04307668 -0.01721256 0.005456097 -0.001190618 ## 6 -0.09047913 0.04307668 -0.01721256 0.005456097 -0.001190618 ## TypeMississippi Treatmentchilled conc ## 1 0 0 95 ## 2 0 0 175 ## 3 0 0 250 ## 4 0 0 350 ## 5 0 0 500 ## 6 0 0 675 ## Plant.L Plant.Q Plant.C Plant^4 Plant^5 Plant^6 ## 79 0.3763089 0.2281037 -0.0418121 -0.3017184 -0.4518689 -0.4627381 ## 80 0.3763089 0.2281037 -0.0418121 -0.3017184 -0.4518689 -0.4627381 ## 81 0.3763089 0.2281037 -0.0418121 -0.3017184 -0.4518689 -0.4627381 ## 82 0.3763089 0.2281037 -0.0418121 -0.3017184 -0.4518689 -0.4627381 ## 83 0.3763089 0.2281037 -0.0418121 -0.3017184 -0.4518689 -0.4627381 ## 84 0.3763089 0.2281037 -0.0418121 -0.3017184 -0.4518689 -0.4627381 ## Plant^7 Plant^8 Plant^9 Plant^10 Plant^11 TypeMississippi ## 79 -0.3701419 -0.2388798 -0.1236175 -0.04910487 -0.0130968 1 ## 80 -0.3701419 -0.2388798 -0.1236175 -0.04910487 -0.0130968 1 ## 81 -0.3701419 -0.2388798 -0.1236175 -0.04910487 -0.0130968 1 ## 82 -0.3701419 -0.2388798 -0.1236175 -0.04910487 -0.0130968 1 ## 83 -0.3701419 -0.2388798 -0.1236175 -0.04910487 -0.0130968 1 ## 84 -0.3701419 -0.2388798 -0.1236175 -0.04910487 -0.0130968 1 ## Treatmentchilled conc ## 79 1 175 ## 80 1 250 ## 81 1 350 ## 82 1 500 ## 83 1 675 ## 84 1 1000

## Second encoder: Target-based

Now, we **present the encoder** discussed in the introduction. It’s a target-based categorical encoder, which uses the correlation between a randomly generated pseudo-target and the real target.

### Construction of a pseudo-target *via* Cholesky decomposition

Most target encoders rely directly on the response variable, which leads to a potential risk called *leakage*. Target encoding is indeed a form of more or less subtle overfitting. Here, in order to somehow circumvent this issue, we use **Cholesky decomposition**. We **create a pseudo-target based on the real target** `uptake`

(centered and scaled, and stored in variable `y`

), and specifically ask that, this pseudo-target has a fixed correlation of `-0.4`

(could be anything) with the response:

# reproducibility seed set.seed(518) # target covariance matrix rho <- -0.4 # desired target C <- matrix(rep(rho, 4), nrow = 2, ncol = 2) diag(C) <- 1 # Cholesky decomposition (C_ <- chol(C)) print(t(C_)%*%C_) X2 <- rnorm(n) XX <- cbind(y, X2) # induce correlation through Cholesky decomposition X_ <- XX %*% C_ colnames(X_) <- c("real_target", "pseudo_target")

Print the **induced correlation** between the randomly generated pseudo-target and the real target:

cor(y, X_[,2]) ## [,1] ## [1,] -0.4008563

Now, a glimpse at `X_`

, a matrix containing the real target and the pseudo target in columns:

print(dim(X_)) print(head(X_)) print(tail(X_)) ## [1] 84 2 ## real_target pseudo_target ## [1,] -1.0368659 -0.6668123 ## [2,] 0.2946905 0.3894672 ## [3,] 0.7015550 0.3485984 ## [4,] 0.9234810 -1.2769424 ## [5,] 0.7477896 -1.1996023 ## [6,] 1.1084194 0.4008157 ## real_target pseudo_target ## [79,] -0.8519275 -0.3455701 ## [80,] -0.8611744 1.7142739 ## [81,] -0.8611744 0.1521795 ## [82,] -0.8611744 -0.3912856 ## [83,] -0.7687052 0.9726421 ## [84,] -0.6762360 0.8791499

### A few checks

By repeating the procedure that we just outlined with 1000 seeds going from `1`

to `1000`

, we obtain a distribution of achieved correlations between the real target and the pseudo target:

## $breaks ## [1] -0.75 -0.70 -0.65 -0.60 -0.55 -0.50 -0.45 -0.40 -0.35 -0.30 -0.25 ## [12] -0.20 -0.15 -0.10 -0.05 ## ## $counts ## [1] 1 0 5 34 74 158 244 227 138 68 31 11 7 2 ## ## $density ## [1] 0.02 0.00 0.10 0.68 1.48 3.16 4.88 4.54 2.76 1.36 0.62 0.22 0.14 0.04 ## ## $mids ## [1] -0.725 -0.675 -0.625 -0.575 -0.525 -0.475 -0.425 -0.375 -0.325 -0.275 ## [11] -0.225 -0.175 -0.125 -0.075 ## ## $xname ## [1] "achieved_correlations" ## ## $equidist ## [1] TRUE ## ## attr(,"class") ## [1] "histogram" print(summary(achieved_correlations)) ## Min. 1st Qu. Median Mean 3rd Qu. Max. ## -0.70120 -0.45510 -0.40270 -0.40040 -0.34820 -0.08723

### Encoding

In order to encode the factors, we use the pseudo-target `y_`

defined as:

y_ <- X_[ , 'pseudo_target']

Our new, numerically encoded covariates are derived by calculating sums of the pseudo-target `y_`

(we could think of other types of aggregations), groupped by factor level for each factor. The new matrix of covariates is named `X_Cholesky`

:

print(dim(X_Cholesky)) print(head(X_Cholesky)) print(tail(X_Cholesky)) ## [1] 84 4 ## Plant Type Treatment conc ## [1,] -1.853112 -18.08574 -7.508514 95 ## [2,] -1.853112 -18.08574 -7.508514 175 ## [3,] -1.853112 -18.08574 -7.508514 250 ## [4,] -1.853112 -18.08574 -7.508514 350 ## [5,] -1.853112 -18.08574 -7.508514 500 ## [6,] -1.853112 -18.08574 -7.508514 675 ## Plant Type Treatment conc ## [79,] 2.628531 9.658954 -0.9182766 175 ## [80,] 2.628531 9.658954 -0.9182766 250 ## [81,] 2.628531 9.658954 -0.9182766 350 ## [82,] 2.628531 9.658954 -0.9182766 500 ## [83,] 2.628531 9.658954 -0.9182766 675 ## [84,] 2.628531 9.658954 -0.9182766 1000

Notice that `X_Cholesky`

has 4 covariates, that `X_onehot`

had 14 covariates, and imagine a situation with a higher cardinality for each factor.

## Fit a model to one-hot encoded and target based covariates

In this section, we compare both types of encoding using cross-validation with Root Mean Squared Errors (RMSE).

### Datasets

# Dataset with one-hot encoded covariates Xy1 <- data.frame(y, X_onehot) # Dataset with pseudo-target-based encoding of covariates Xy2 <- data.frame(y, X_Cholesky)

### Comparison

Using a Random Forest here as a simple illustration without hyperparameter tuning, but tree-based models will typically handle this type of data. Not linear models, nor Neural Networks or Support Vector Machines.

Random Forests with 100 seeds, going from `1`

to `100`

are adjusted:

n_reps <- 100 n_train <- length(y) `%op%` <- foreach::`%do%` pb <- utils::txtProgressBar(min=0, max=n_reps, style = 3) errs <- foreach::foreach(i = 1:n_reps, .combine=rbind)%op% { # utils::setTxtProgressBar(pb, i) set.seed(i) index_train <- sample.int(n_train, size = floor(0.8*n_train)) obj1 <- randomForest(y ~ ., data=Xy1[index_train, ]) obj2 <- randomForest(y ~ ., data=Xy2[index_train, ]) c(sqrt(mean((predict(obj1, newdata=as.matrix(Xy1[-index_train, -1])) - y[-index_train])^2)), sqrt(mean((predict(obj2, newdata=as.matrix(Xy2[-index_train, -1])) - y[-index_train])^2))) } close(pb) colnames(errs) <- c("one-hot", "target-based") print(colMeans(errs)) print(apply(errs, 2, sd)) print(sapply(1:2, function (j) summary(errs[,j]))) ## one-hot target-based ## 0.4121657 0.4574857 ## one-hot target-based ## 0.09710344 0.07584037 ## [,1] [,2] ## Min. 0.1877 0.2850 ## 1st Qu. 0.3566 0.4039 ## Median 0.4037 0.4464 ## Mean 0.4122 0.4575 ## 3rd Qu. 0.4784 0.4913 ## Max. 0.6470 0.6840

There are certainly some improvements to be brought to this methodology, but the results discussed in this post already look quite encouraging to me.

**Note:** I am currently looking for a *gig*. You can hire me on Malt or send me an email: **thierry dot moudiki at pm dot me**. I can do descriptive statistics, data preparation, feature engineering, model calibration, training and validation, and model outputs’ interpretation. I am fluent in Python, R, SQL, Microsoft Excel, Visual Basic (among others) and French. My résumé? Here!

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