Hyperparameter tuning is one of the most important factors that can improve the accuracy of a gradient boosting model. Some key hyperparameters that often need tuning include the number of iterations/trees, learning rate, maximum depth of each tree, minimum observations in the leaf nodes, and tree pruning parameters. Finding the optimal configuration of these hyperparameters requires grid searching through different values either manually or using automated techniques like randomized search. The right combination of hyperparameters can help the model strike the right balance between underfitting and overfitting to the training data.

Using more feature engineering to extract additional informative features from the raw data can provide the gradient boosting model with more signals to learn from. Although gradient boosting models can automatically learn interactions between features, carefully crafting transformed features based on domain knowledge can vastly improve a model’s ability to find meaningful patterns. This may involve discretizing continuous variables, constructing aggregated features, imputing missing values sensibly, etc. More predictive features allow the model to better separate different classes/targets.

Leveraging ensemble techniques like stacking can help boost accuracy. Stacking involves training multiple gradient boosting models either on different feature subsets/transformations or using different hyperparameter configurations, and then combining their predictions either linearly or through another learner. This ensemble approach helps address the variance present in any single model, leading to more robust and generalized predictions. Similarly, random subspace modeling, where each model is trained on a random sample of features, can reduce variability.

Using more training data, if available, often leads to better results with gradient boosting models since they are data-hungry algorithms. Collecting more labeled examples allows the models to learn more subtle and complex patterns in large datasets. Simply adding more unlabeled data may not always help; the data need to be informative for the task. Also, addressing any class imbalance issues in the training data can enhance model performance. Strategies like oversampling the minority class may be needed.

Choosing the right loss function suited for the problem is another factor. While deviance/misclassification error works best for classification, other losses like Huber/quantilic optimize other objectives better. Similarly, different tweaks like softening class probabilities with logistic regression in the final stage can refine predictions. Architectural choices like using more than one output unit enable multi-output or multilabel learning. The right loss function guides the model to learn patterns optimally for the problem.

Carefully evaluating feature importance scores and looking for highly correlated or redundant features can help remove non-influential features pre-processing. This “feature selection” step simplifies the learning process and prevents the model from wasting capacity on unnecessary features. It may even improve generalization by reducing the risk of overfitting to statistical noise in uninformative features. Similarly, examining learned tree structures can provide intuition on useful transformations and interactions to be added.

Using other regularization techniques like limiting the number of leaves in each individual regression tree or adding an L1 or L2 penalty on the leaf weights in addition to shrinkage via learning rate can guard against overfitting further. Tuning these regularization hyperparameters appropriately allows achieving the optimal bias-variance tradeoff for maximum accuracy on test data over time.

Hyperparameter tuning, feature engineering, ensemble techniques, larger training data, proper loss function selection, feature selection, regularization, and evaluating intermediate results are some of the key factors that if addressed systematically can significantly improve the test accuracy of gradient boosting models on complex problems by alleviating overfitting and enhancing their ability to learn meaningful patterns from data.