The `reg_lambda`

parameter in XGBoost is an alias for the `lambda`

parameter, which controls the L2 regularization term on weights. By adjusting `reg_lambda`

, you can influence the model’s complexity and its ability to generalize.

```
from xgboost import XGBClassifier
from sklearn.datasets import make_classification
from sklearn.model_selection import train_test_split
# Generate synthetic data
X, y = make_classification(n_samples=1000, n_features=20, n_informative=2, n_redundant=10, random_state=42)
# Split the dataset into training and test sets
X_train, X_test, y_train, y_test = train_test_split(X, y, test_size=0.2, random_state=42)
# Initialize the XGBoost classifier with a lambda value
model = XGBClassifier(reg_lambda=0.5, eval_metric='logloss')
# Fit the model
model.fit(X_train, y_train)
# Make predictions
predictions = model.predict(X_test)
```

As discussed in the example on configuring the `lambda`

parameter, `reg_lambda`

determines the strength of the L2 regularization term on the weights in the XGBoost model. It is a regularization parameter that can help prevent overfitting by adding a penalty term to the objective function, which discourages large weights. `reg_lambda`

accepts non-negative values, and the default value in XGBoost is 1.

To recap, the key points when configuring the `reg_lambda`

parameter are:

- Valid range: Non-negative values
- Default value: 1
- Impact on model complexity:
- Higher values increase the regularization strength, which can help prevent overfitting but may lead to underfitting if set too high
- Lower values reduce the regularization strength, allowing the model to fit the training data more closely but potentially increasing the risk of overfitting

- Interaction with other regularization parameters:
`reg_lambda`

works in conjunction with`alpha`

(L1 regularization) to control model complexity- Tuning these parameters together can help find the right balance between overfitting and underfitting

For practical guidance on choosing the right `reg_lambda`

value, refer to the tip on configuring the `lambda`

parameter.