add mention of multiple treatments + citation

Author: drbenvincent

docs/source/notebooks/graded_intervention_time_series_single_channel_ols.ipynb

@@ -6,7 +6,7 @@
    "source": [
     "# Graded Intervention Time Series (OLS)\n",
     "\n",
-    "**Graded Intervention Time Series** extends classical interrupted time series analysis to handle **graded interventions** - policies or treatments with varying intensity over time, rather than simple on/off changes. Traditional ITS methods model binary interventions (e.g., \"policy enacted\" vs \"no policy\"). This method (technically called Transfer Function Interrupted Time Series or TF-ITS in the literature {cite:p}`box1975intervention`) handles more realistic scenarios where:\n",
+    "**Graded Intervention Time Series** extends classical interrupted time series analysis to handle **graded interventions** - policies or treatments with varying intensity over time, rather than simple on/off changes. Traditional ITS methods model binary interventions (e.g., \"policy enacted\" vs \"no policy\"). This method (technically called Transfer Function Interrupted Time Series or TF-ITS in the literature {cite:p}`box1975intervention`, with extensions to multiple time series by {cite:p}`abraham1980intervention`) handles more realistic scenarios where:\n",
     "\n",
     "1. **Intervention intensity varies continuously** (e.g., advertising spend \\$0 - 100k, communication frequency 0-10 messages/week)\n",
     "2. **Effects saturate** - diminishing returns as exposure increases (10th message less impactful than the 1st)\n",
@@ -28,6 +28,10 @@
     "- ✅ Reason to expect **saturation** (diminishing returns) or **carryover effects** (persistence)\n",
     "- ✅ Baseline controls available for confounders\n",
     "\n",
+    ":::{note}\n",
+    "This notebook demonstrates the single channel (single time series) case. The transfer function intervention analysis framework extends naturally to multiple time series {cite:p}`abraham1980intervention`, but this extension is not yet implemented in CausalPy.\n",
+    ":::\n",
+    "\n",
     "Compare to related methods:\n",
     "- **Classic {term}`Interrupted Time Series`**: Binary on/off intervention (no dose-response modeling)\n",
     "- **{term}`Synthetic Control`**: Multiple control units available for comparison\n",

docs/source/references.bib

@@ -194,6 +194,17 @@ @article{GriffithCollider
   year={2020},
   publisher={Nature}
 }
+@article{abraham1980intervention,
+  title={Intervention analysis and multiple time series},
+  author={Abraham, Bovas},
+  journal={Biometrika},
+  volume={67},
+  number={1},
+  pages={73--78},
+  year={1980},
+  publisher={Oxford University Press}
+}
+
 @article{box1975intervention,
   title={Intervention analysis with applications to economic and environmental problems},
   author={Box, George EP and Tiao, George C},