Esaminare la struttura degli input categorici

In questo esercizio userai model.matrix() (docs) per esaminare come R rappresenta, ai fini del modeling, dati con input sia categorici sia numerici. Il dataset flowers (derivato dal pacchetto Sleuth3) è già stato caricato. Contiene le seguenti colonne:

Flowers: il numero medio di fiori su una pianta di meadowfoam
Intensity: l’intensità di un trattamento luminoso applicato alla pianta
Time: una variabile categorica — quando (Late o Early) nel ciclo di vita è avvenuto il trattamento luminoso

L’obiettivo finale è prevedere Flowers in funzione di Time e Intensity.

Questo esercizio fa parte del corso

Supervised Learning in R: Regression

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Istruzioni dell'esercizio

Chiama la funzione str() su flowers per vedere i tipi di ciascuna colonna.
Usa la funzione unique() sulla colonna flowers$Time per vedere i possibili valori assunti da Time. Quanti valori unici ci sono?
Crea una formula per esprimere Flowers in funzione di Intensity e Time. Assegnala alla variabile fmla e stampala.
Usa fmla e model.matrix() per creare la matrice del modello per il data frame flowers. Assegnala alla variabile mmat.
Usa head() per esaminare le prime 20 righe di flowers.
Ora esamina le prime 20 righe di mmat.
- La colonna numerica Intensity è diversa?
- Che cosa è successo alla colonna categorica Time di flowers?
- Come è rappresentato Time == 'Early'? E Time == 'Late'?

Esercizio pratico interattivo

Prova a risolvere questo esercizio completando il codice di esempio.

# Call str on flowers to see the types of each column
___

# Use unique() to see how many possible values Time takes
___

# Build and print a formula to express Flowers as a function of Intensity and Time: fmla
(fmla <- ___("Flowers ~ Intensity + Time"))

# Use fmla and model.matrix to see how the data is represented for modeling
mmat <- ___

# Examine the first 20 lines of flowers
___

# Examine the first 20 lines of mmat
___

Modifica ed esegui il codice

Questo esercizio fa parte del corso

Supervised Learning in R: Regression

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In this chapter we introduce the concept of regression from a machine learning point of view. We will present the fundamental regression method: linear regression. We will show how to fit a linear regression model and to make predictions from the model.

Exercise 1: Welcome and Introduction Exercise 2: Identify the regression tasks Exercise 3: Linear regression - the fundamental method Exercise 4: Code a simple one-variable regression Exercise 5: Examining a model Exercise 6: Predicting once you fit a model Exercise 7: Predicting from the unemployment model Exercise 8: Multivariate linear regression (Part 1)Exercise 9: Multivariate linear regression (Part 2)Exercise 10: Wrapping up linear regression

Now that we have learned how to fit basic linear regression models, we will learn how to evaluate how well our models perform. We will review evaluating a model graphically, and look at two basic metrics for regression models. We will also learn how to train a model that will perform well in the wild, not just on training data. Although we will demonstrate these techniques using linear regression, all these concepts apply to models fit with any regression algorithm.

Exercise 1: Evaluating a model graphically Exercise 2: Graphically evaluate the unemployment model Exercise 3: The gain curve to evaluate the unemployment model Exercise 4: Root Mean Squared Error (RMSE)Exercise 5: Calculate RMSE Exercise 6: R-squared Exercise 7: Calculate R-squared Exercise 8: Correlation and R-squared Exercise 9: Properly Training a Model Exercise 10: Generating a random test/train split Exercise 11: Train a model using test/train split Exercise 12: Evaluate a model using test/train split Exercise 13: Create a cross validation plan Exercise 14: Evaluate a modeling procedure using n-fold cross-validation

Before moving on to more sophisticated regression techniques, we will look at some other modeling issues: modeling with categorical inputs, interactions between variables, and when you might consider transforming inputs and outputs before modeling. While more sophisticated regression techniques manage some of these issues automatically, it's important to be aware of them, in order to understand which methods best handle various issues -- and which issues you must still manage yourself.

Exercise 1: Variabili categoriche in input Exercise 2: Esaminare la struttura degli input categorici

Esercizio in corso

Exercise 3: Modellare con input categorici Exercise 4: Interazioni Exercise 5: Modellare un'interazione Exercise 6: Modeling an interaction (2)Exercise 7: Trasformare la risposta prima della modellazione Exercise 8: Errore relativo Exercise 9: Modellare un output monetario trasformato con log Exercise 10: Confrontare RMSE e root-mean-squared Relative Error Exercise 11: Trasformare gli input prima della modellazione Exercise 12: Trasformazioni degli input: la "mazza da hockey"Exercise 13: Trasformazioni degli input: la "hockey stick" (2)

Now that we have mastered linear models, we will begin to look at techniques for modeling situations that don't meet the assumptions of linearity. This includes predicting probabilities and frequencies (values bounded between 0 and 1); predicting counts (nonnegative integer values, and associated rates); and responses that have a non-linear but additive relationship to the inputs. These algorithms are variations on the standard linear model.

Exercise 1: Logistic regression to predict probabilities Exercise 2: Fit a model of sparrow survival probability Exercise 3: Predict sparrow survival Exercise 4: Poisson and quasipoisson regression to predict counts Exercise 5: Poisson or quasipoisson Exercise 6: Fit a model to predict bike rental counts Exercise 7: Predict bike rentals on new data Exercise 8: Visualize the bike rental predictions Exercise 9: GAM to learn non-linear transforms Exercise 10: Writing formulas for GAM models Exercise 11: Writing formulas for GAM models (2)Exercise 12: Model soybean growth with GAM Exercise 13: Predict with the soybean model on test data

In this chapter we will look at modeling algorithms that do not assume linearity or additivity, and that can learn limited types of interactions among input variables. These algorithms are *tree-based* methods that work by combining ensembles of *decision trees* that are learned from the training data.

Exercise 1: The intuition behind tree-based methods Exercise 2: Predicting with a decision tree Exercise 3: Random forests Exercise 4: Build a random forest model for bike rentals Exercise 5: Predict bike rentals with the random forest model Exercise 6: Visualize random forest bike model predictions Exercise 7: One-Hot-Encoding Categorical Variables Exercise 8: vtreat on a small example Exercise 9: Novel levels Exercise 10: vtreat the bike rental data Exercise 11: Gradient boosting machines Exercise 12: Find the right number of trees for a gradient boosting machine Exercise 13: Fit an xgboost bike rental model and predict Exercise 14: Evaluate the xgboost bike rental model Exercise 15: Visualize the xgboost bike rental model