diff --git a/environment.yml b/environment.yml
index ac6af78..a9f6047 100644
--- a/environment.yml
+++ b/environment.yml
@@ -28,6 +28,7 @@ dependencies:
   - requests=2.28  # used only in chapter 19 for REST API queries
   - scikit-learn=1.1  # machine learning library
   - scipy=1.9  # scientific/technical computing library
+  - statsmodels=0.13  # used only in chapter 15 for time series analysis
   - tqdm=4.64  # a progress bar library
   - wheel  # built-package format for pip
   - widgetsnbextension=4.0  # interactive HTML widgets for Jupyter notebooks
diff --git a/requirements.txt b/requirements.txt
index ea33758..ee1556c 100644
--- a/requirements.txt
+++ b/requirements.txt
@@ -96,3 +96,6 @@ ipywidgets~=8.0.2
 
 # Optional: pydot is only used in chapter 10 for tf.keras.utils.plot_model()
 pydot~=1.4.2
+
+# Optional: statsmodels is only used in chapter 15 for time series analysis
+statsmodels~=0.13.2