Comment choisir les bonnes fonctionnalités pour votre modèle de machine learning ?

Feature redundance can be especially problematic. It can cause (i) multicollinearity (when combined input features can predict another input feature), destabilizing the correlation coefficients, (ii) overfitting (when the ML memorizes training data and thus doesn’t generalize to new datasets), (iii) increased dimensionality of the problem, requiring longer training times and higher computational needs (esp. problematic where matrix inversion is used in algorithms like linear regression), and, (iv) model performance to suffer due to preferential selection of some features over others, potentially resulting in less robust models. Feature Selection, Dimensionality Reduction techniques (PCA), and LASSO regularization can address these issues.

Selecting the optimal features for a machine learning model is a critical process, pivotal in mitigating bias and enhancing prediction accuracy. Initially, comprehending the problem domain and its business implications is imperative, guiding the identification of pertinent features. Employing exploratory data analysis techniques such as correlation analysis, visualizations, and basic statistics aids in understanding feature relevance and interrelationships. Further, employing methods like feature importance analysis and dimensionality reduction techniques like PCA helps. Eliminating irrelevant or redundant features, using regularization, and iteratively evaluating model through cross-validation refine the feature selection process.

You have to begin by comprehensively exploring the dataset, examining its structure, distribution, and relationships between variables. Use statistical techniques and visualization tools to identify patterns, outliers, and correlations. Consider domain knowledge to determine which features are relevant to the problem at hand. Additionally, employ techniques like feature importance ranking, dimensionality reduction, and feature engineering to enhance model performance. Continuously evaluate and iterate on feature selection based on model performance metrics and domain-specific insights.

Exploratory data analysis always helps in getting hold of data but in my experience understanding the domain is also crucial; domain experts can provide insights into which features are likely to be relevant. This foundational knowledge guides you in making informed decisions about which features to consider in your model.

In my experience, taking the time to visualize and explore your data can be incredibly valuable. Not only can it help you identify patterns and trends, but it can also reveal potential issues or biases in your data that could impact the performance of your model. One approach I've found helpful is to create a "feature map" - a simple diagram that shows how each feature relates to the target outcome and to other features in your dataset. This helps to identify potential correlations and redundancies, which can then guide your decision-making process. Taking the time to thoroughly understand your data is essential for making informed decisions about which features to include in your model.

Here is a practical way to understand feature importance. Open Google Colab. Install 'lightgbm' and 'ucimlrepo'. Use fetch_ucirepo(id=544) to fetch the "Obesity Levels" dataset. Drop the "Weight" and "Height" columns. Add a "Random" feature with random values. Process categorical columns as needed. Split the data into training and testing sets. Train a LightGBM model with your training data. Extract and visualize feature importance using split and gain methods. Finally, identify features with lower importance than the 'Random' feature. Apply permutation importance by shuffling each feature and measuring the drop in model accuracy. So, which features contribute to predicting obesity levels and which features can be excluded?

By using GridSearch it is possible to identify the best features for the model. Also, when applying PCA it is also possible to reduce the dimension of the data and capture 90% variance in the main variables.

I completely agree with using techniques like Random Forest or Gradient Boosting to evaluate the significance of each feature in your dataset. In my experience, it's also important to consider the domain knowledge and business context of your problem when evaluating feature importance. For example, if you're building a model to predict customer churn, you may want to prioritize features that are directly related to customer behavior, such as purchase history or usage patterns. By combining these insights with the feature importance scores from your algorithms, you can make more informed decisions about which features to include in your final model.

Domain-specific expertise is critical to proper feature selection. For instance, in the investment industry, hedge funds have tried to apply ML techniques to improve systematic investment strategies for decades, but candidates with deep investment expertise and a strong understanding of machine learning is difficult. Using quants with adv. degrees in CS & math is common, but these candidates are ill-equipped. Exploring spot pricing will not reveal critical insights to build decent models. Non-stationarity among features must be incorporated into any/all models. Market microstructures change with time (passive influence breaks correlation w/fundamentals) & options’ markets impact on spot prices changes cross-asset correlations.

Techniques like Random Forest and Gradient Boosting provide scores showing how useful each feature is. It's smart to start with a basic model, then gradually add features one by one to see how they improve performance. This step-by-step process makes it clear which features have the biggest positive impact. For example, imagine predicting house prices. Features like square footage, number of bedrooms, and location would likely be very important. But other factors like window type or roof color probably wouldn't matter much. The importance scores help reveal the key drivers.

Selecting the right features for a machine learning model involves several key steps. First, I understand the problem domain and collaborate with domain experts to identify relevant features. Then, I conduct exploratory data analysis to assess feature distributions and relationships. Feature engineering is used to create new informative features. I prioritize features based on their predictive strength through methods like correlation analysis and feature importance ranking. Automated techniques like forward selection or regularization help manage high-dimensional data. Finally, I validate selected features using cross-validation to ensure generalizability.

Data preparation is a make-or-break step before feature selection. Missing data needs handling - imputing values, removing rows, etc. Outliers that are abnormally high or low can really skew things, so you may need to cap, remove, or adjust them. Features on different scales (e.g. age 0-100 vs. income $10k-$1M) require normalization/scaling so one doesn't dominate. Categorical data like colors, countries, or product names also require special encoding to numeric form that algorithms understand. Popular techniques are one-hot encoding (creating binary flags) or label encoding (assigning numbers).

Proper data preparation is critical for feature selection. Start by addressing missing values through imputation methods like mean substitution or more sophisticated techniques like K-Nearest Neighbors. Handle outliers by removing them or using transformation methods. Normalize or scale features so they are on the same scale, which is essential for algorithms like SVMs or K-means. For categorical data, use encoding methods like one-hot encoding for nominal variables or label encoding for ordinal variables. For example, in a housing price model, encode categorical features like neighborhood or house style to numeric form.

In machine learning, feature selection is critical for model performance. I use to leverage Principal Component Analysis (PCA) for linear data and Uniform Manifold Approximation and Projection (UMAP) for nonlinear data. PCA uncovers linear relationships by transforming features into orthogonal components, while UMAP preserves local and global structures in nonlinear data. I often employ both techniques sequentially or in tandem for datasets with mixed structures. Integrating PCA and UMAP ensures comprehensive feature selection, enhancing model robustness and interpretability.

Techniques like PCA or LDA can help you transform your features into a lower-dimensional space, which can improve model performance by reducing noise and overfitting. This is where domain knowledge comes into play underlying relationships between the original features can help you identify the most important dimensions to retain. In my experience, a combination of dimensionality reduction and feature selection techniques (such as mutual information or recursive feature elimination) can be particularly effective in identifying the most relevant features for your model.

Dimensionality reduction techniques like PCA and LDA can be really helpful when you have tons of features - maybe thousands or more. They find smart ways to combine and compact all that info into just a handful of new "summary" features that pack most of the predictive punch. Imagine predicting real estate prices using details on every room, appliance, fixture, etc. That's potentially thousands of features! PCA/LDA could distill that down to maybe 10-20 concise features capturing the most important property characteristics in a simplified way.

Dimensionality reduction techniques like PCA and LDA are useful for simplifying your feature set while retaining important information. PCA transforms features into a set of orthogonal components that capture the most variance, while LDA focuses on maximizing class separability. For example, in image recognition, PCA can reduce thousands of pixel features to a manageable number of principal components, preserving critical visual patterns. Although these new features are composites of the originals and might be harder to interpret, they can significantly enhance model performance by reducing noise and mitigating overfitting. Use these methods to streamline data and improve predictive accuracy.

Regularization methods in feature reduction aim to prevent overfitting by penalizing the complexity of the model. 1. Lasso Regression (L1 Regularization) Adds the absolute value of the coefficients as a penalty term to the loss function.Encourages sparsity by forcing some coefficients to become exactly zero, effectively performing feature selection.Useful when dealing with a large number of features and wanting to automatically select the most important ones. 2.Ridge Regression (L2 Regularization)Adds the squared magnitude of the coefficients as a penalty term to the loss function. Encourages smaller coefficient values, but doesn't force them to zero.Helps to reduce the impact of multicollinearity among features and stabilizes the model.

Regularization methods, like Lasso, Ridge, and Elastic Net, help prevent overfitting and improve robustness. Lasso selects features by adding an L1 penalty term, Ridge shrinks coefficients with an L2 penalty, and Elastic Net combines both. Other techniques like dropout, early stopping, and cross-validation aid in feature selection. Domain knowledge and data analysis guide feature relevance, interpretability, and efficiency.

Regularization methods like LASSO and Ridge regression are essential for feature selection in models with many features. LASSO adds a penalty that can shrink the coefficients of less important features to zero, effectively removing them from the model. Ridge regression, while similar, penalizes the coefficients without necessarily shrinking any to zero, helping to manage multicollinearity. For example, in a marketing model predicting sales, LASSO might eliminate irrelevant demographic features, focusing only on key predictors like advertising spend and seasonal trends. These methods simplify models, reduce overfitting, and highlight the most impactful features, enhancing model performance and interpretability.

In the investment industry, the majority of Algo Funds marketed to retail are significantly overfit, showing excellence performance stats (win ratios, avg gain/trade, Sharpe, volatility), but these statistics are only good because the backtest results used to market the strategy are the very data on which they were trained! Models are overfit when they de facto memorize the training set, thus they don’t generalize. In financial markets, consider a fund that sells $VIX calls (nothing else) and is marketed with back test performance since June 2018. Of course they don’t mention $VIX went through the roof in February 2018, taking funds like this one out with it in one swoop, an event now called the “2018 Volmageddon.”

I've found that LASSO is particularly effective in this regard. By shrinking the coefficients of less important features to zero, it's like having a "feature editor" built right into your model. In these situations, LASSO can help you distill down to the most relevant and informative features, which can lead to better predictive performance and fewer false positives. Avoid overfitting, which can occur when your model becomes too complex and starts to fit the noise in the data rather than the underlying patterns. Overall, I think regularization techniques like LASSO and Ridge regression are essential tools for any machine learning practitioner looking to build robust and accurate models.

Here’s a summary of the feature selection methods available in the scikit-learn (sklearn) library: - Recursive Feature Elimination (RFE): A widely used feature selection algorithm that iteratively eliminates the least significant features until the desired number of features is achieved. - Sequential Feature Selection (SFS): A greedy algorithm that iteratively adds or removes features from a dataset to improve predictive model performance. - Univariate Feature Selection: methods like SelectKBest and SelectPercentile that utilize statistical tests (such as F-test and chi-squared) to select the most informative features. - Removing Features with Low Variance (VarianceThreshold)

You could try gauging the relevance of the features. While following most of the steps mentioned in this article, you can work on finding out how causal these features are with the other features. Try using granger causality for this. This can help you understand how important one feature is with respect to the other. But again, keep in mind that this technique is for time series data.