If you are asking this, then you probably do not understand the algorithm you are using. This is a bad habit to start with, but if you do not want, have the time or the interest, the following table should be a decent starting point.

# Some definitions

**Centering** Centering a variable consists in substracting the mean value to each value, so that the new variable has a sample mean equals to 0.

**Reducing** Reducing a value consists in dividing every value of the sample by the standard deviation of the sample.

**Scaling** Here we will call “scaling” the action consisting of centering the data and then reducing it. After the scaling, the sample has a null sample mean and a standard deviation of 1.

## Generalities about algorithms regarding the scaling of the data

*Supervised learning*

Algorithm | Scaling |
---|---|

Decision Tree | No |

Random Forest | No |

Gradient Boosting | No |

Linear Regression | No |

Penalized Linear Regression | Yes, probably |

SVM (Kernel) | Yes, probably |

k-Nearest Neighbours | Yes, probably |

Nearest centroid | Yes, probably |

Neural Network | Yes, probably |

*Unsupervised learning*

Algorithm | Scaling needed |
---|---|

PCA | Yes, probably |

Random projections | Yes, probably |

t-SNE | Yes, probably |

The following tables should be read this way. If scaling is not needed, it means you should not see changes between the results you obtain with or without scaling.

If it says yes, probably, it means that scaling is useful as features should have the same order of magnitude for the algorithm to work properly. However, it does not mean that performance will increase.

Per example, in presence of features lying on a bounded scale (when translating an image to a grayscale image and then feeding it to a neural network, or when turning a text to a TFIDF matrix), scaling is not recommended.

## When the scaling is performed before applying the algorithm

Note that some libraries (especially in R) take care of the scaling before applying the algorithm. Though this seems to be a bad idea (the behaviour of the algorithm if a column is constant becomes implementation dependent, per example), this may save you some efforts.

```
svm(x, y = NULL, scale = TRUE, type = NULL, kernel =
"radial", degree = 3, gamma = if (is.vector(x)) 1 else 1 / ncol(x),
coef0 = 0, cost = 1, nu = 0.5,
class.weights = NULL, cachesize = 40, tolerance = 0.001, epsilon = 0.1,
shrinking = TRUE, cross = 0, probability = FALSE, fitted = TRUE,
..., subset, na.action = na.omit)
```

This is the `svm`

function as presented in the `e1071`

R package. Note the default value of `scale`

.

```
glmnet(x, y, family=c("gaussian","binomial","poisson","multinomial","cox","mgaussian"),
weights, offset=NULL, alpha = 1, nlambda = 100,
lambda.min.ratio = ifelse(nobs<nvars,0.01,0.0001), lambda=NULL,
standardize = TRUE, intercept=TRUE, thresh = 1e-07, dfmax = nvars + 1,
pmax = min(dfmax * 2+20, nvars), exclude, penalty.factor = rep(1, nvars),
lower.limits=-Inf, upper.limits=Inf, maxit=100000,
type.gaussian=ifelse(nvars<500,"covariance","naive"),
type.logistic=c("Newton","modified.Newton"),
standardize.response=FALSE, type.multinomial=c("ungrouped","grouped"))
```

In the `glmnet`

package the argument is now called `standardize`

. Note that here, the response can be standardized as well. This topic will not be covered in this post. Now coming back to the dangers of such an approach, look at the following sample:

```
N <- 100
P <- 5
X <- matrix(data = rnorm(N*P), nrow = N)
Y <- matrix(rnorm(N), nrow = N)
X[,1] <- X[,1]*0 # some evil action
require("glmnet")
model <- glmnet(x = X, y = Y)
# Runs without issues
require("e1071")
model2 <- svm(x = X, y = Y)
# Warning message:
# In svm.default(x = X, y = Y) :
# Variable(s) ‘X1’ constant. Cannot scale data.
```

In the case where you run many models on many datasets (or many combination of features) some will scale the data, others will not (if one of the features is constant) and may report bad performances because the scaling was not operated…

## Is it always possible to scale the data ?

### Theoretical point of view

The assumptions when substracting the mean and dividing by the standard deviation is that they both exist ! Though with finite samples, we can always evaluate sample mean and sample variance, if the variables come from (say a Cauchy distribution) the coefficients for scaling may vary dramatically when enriching the sample with new points.

However, in the case where one is trying to learn anything from distributions that are not integrable, there will be many other issues to deal with.

### Practical point of view

With a sparse dataset, scaling is not a good idea : it would force many of the points (the ones that are 0s in the original dataset). But reducing the variables is possible! And it turns out that some algorithms are not affected by the centering (or not) of the data.

## A better approach

As we saw, there are actually three types of algorithms : those who do not change with monotonic transformations of the inputs, those who do not change with translations of the input and those who do not fit in the first two categories.

Note that the “monotonic transformation invariance” is the strongest property, as translation is just a monotonic transformation.

So the algorithms would enjoy a better representation in this table:

*Supervised learning*

Algorithm | Translation invariant | Monotonic transformation invariant |
---|---|---|

Decision Tree | X | X |

Random Forest | X | X |

Gradient Boosting | X | X |

Linear Regression | X | |

Penalized Linear Regression | ||

SVM (Gaussian kernel) | X | |

SVM (Other kernels) | ||

k-Nearest Neighbours | X | |

Nearest centroid | X | |

Neural Network |

*Unsupervised learning*

Algorithm | Translation invariant | Monotonic transformation invariant |
---|---|---|

PCA | ||

Random projections | ||

t-SNE | X |

# Learning more

The elements of statistical learning by Trevor Hastie, Robert Tibshirani, Jerome Friedman is a brilliant introduction to the topic and will help you have a better understanding of most of the algorithms presented in this article !