add reference + citation to helfenstein1991use

Author: drbenvincent

docs/source/notebooks/graded_intervention_time_series_single_channel_ols.ipynb

@@ -12,6 +12,8 @@
     "2. **Effects saturate** - diminishing returns as exposure increases (10th message less impactful than the 1st)\n",
     "3. **Effects persist over time** - past interventions continue to influence outcomes (behavioral habits change gradually)\n",
     "\n",
+    "For a good introductory overview of transfer function models and intervention analysis, see {cite:p}`helfenstein1991use`.\n",
+    "\n",
     "### Key Components\n",
     "\n",
     "- **Saturation transforms**: Model diminishing returns using Hill, logistic, or Michaelis-Menten functions\n",

docs/source/references.bib

@@ -59,6 +59,17 @@ @book{hansenEconometrics
   publisher={Princeton}
 }
 
+@article{helfenstein1991use,
+  title={The use of transfer function models, intervention analysis and related time series methods in epidemiology},
+  author={Helfenstein, Ulrich},
+  journal={International journal of epidemiology},
+  volume={20},
+  number={3},
+  pages={808--815},
+  year={1991},
+  publisher={Oxford University Press}
+}
+
 @book{aronowFoundations,
   author={Aronow, P and  Miller, B},
   title={Foundations of Agnostic Statistics},