diff --git a/ML/1-data-exploration.R b/ML/1-data-exploration.R
index b5cdca2..1b77964 100644
--- a/ML/1-data-exploration.R
+++ b/ML/1-data-exploration.R
@@ -160,7 +160,6 @@ gp2$ggsave(
   )
 
 
-
 # save-data ---------------------------------------------------------------
 
 ds1 %>% readr$write_rds(here("ML","data-unshared","model_data.RDS"))
diff --git a/chapter3.qmd b/chapter3.qmd
index c395065..540be4e 100644
--- a/chapter3.qmd
+++ b/chapter3.qmd
@@ -1,3 +1,7 @@
 # Methods
 
 ## Population and Data
+
+This study was designed using the Medical Information Mart for Intensive Care (MIMIC) database [@johnsonalistair]. MIMIC (Medical Information Mart for Intensive Care) is an extensive, freely-available database comprising de-identified health-related data from patients who were admitted to the critical care units of the Beth Israel Deaconess Medical Center. The study is using version IV of the database, containing data from 2008 - 2019. The database contains many different types of information, but only data from the patients and laboratory events table are used in this study.
+
+## Data Variables and Outcomes
diff --git a/references.bib b/references.bib
index fbcc603..8d19fb3 100644
--- a/references.bib
+++ b/references.bib
@@ -290,3 +290,13 @@ Publisher: Endeavor Business Media},
 	url = {https://linkinghub.elsevier.com/retrieve/pii/S1875918115000343},
 	langid = {en}
 }
+
+@article{johnsonalistair,
+	title = {MIMIC-IV},
+	author = {{Johnson, Alistair},  and {Bulgarelli, Lucas},  and {Pollard, Tom},  and {Horng, Steven},  and {Celi, Leo Anthony},  and {Mark, Roger}, },
+	doi = {10.13026/S6N6-XD98},
+	url = {https://physionet.org/content/mimiciv/1.0/},
+	note = {Version Number: 1.0
+Type: dataset
+DOI: 10.13026/S6N6-XD98}
+}