add info about identifiers used

website/content/data/overview.md

@@ -14,17 +14,20 @@ toc = false
 # Mapping file
 
 A records file, `cxr-record-list.csv.gz`, provides a mapping between the image (dicom_id), the study (study_id), and the patient (subject_id).
-
 Another records file, `cxr-study-list.csv.gz`, provides a mapping between the studies (`study_id`) and patients (`subject_id`).
 
+All patient identifiers begin with the digit 1 and have a total length of 8 digits.
+All study identifiers begin with the digit 5 and have a total length of 8 digits.
+DICOM file names are unique 40 character hexadecimal strings with dashes separating groups of eight characters.
+
 Images are provided in DICOM format; see the [image](/image) section for more information about the images.
 
 Reports are provided as plain text files; see the [reports](/reports) section for more information about the reports.
 
 # Data Organization
 
 Data files are made available in a hierarchical strcture.
-The following block lists first patient's records as an demonstrative example (MIMIC-CXR v2.0.0):
+The following block lists the first patient's records as an demonstrative example (MIMIC-CXR v2.0.0):
 
 
 ```