Update README.md

README.md

@@ -25,6 +25,7 @@ There are currently few datasets appropriate for training and evaluating models
   7. What was the point of the brown v board of education?
   8. What were the main arguments?
   9. Why is it important today?
+  10. 
 
 ### Collection
  * The corpus is a combination of three standard TREC collections: MARCO Ranking passages, Wikipedia (TREC CAR), and News (Washington Post)
@@ -32,6 +33,13 @@ There are currently few datasets appropriate for training and evaluating models
  * The [TREC CAR paragraph collection v2.0](http://trec-car.cs.unh.edu/datareleases/v2.0/paragraphCorpus.v2.0.tar.xz)
  * The [TREC Washington Post Corpus version 2](https://ir.nist.gov/wapo/WashingtonPost.v2.tar.gz): Note this is behind a password and requires an organizational agreement, to obtain it see: https://ir.nist.gov/wapo/
 
+
+### Document ID format
+ * The document id format is `[collection_id_paragraph_id]` with collection id and paragraph id separated by an underscore.
+ * The collection ids are in the set: `{MARCO, CAR, WAPO}`. 
+ * The paragraph ids are: standard provided by MARCO and CAR. For WAPO the paragraph ID is `[article_id-paragraph_index]` where the paragraph_index is the *starting from 1-based* index of the paragraph using the provided paragraph markup separated by a single dash. 
+ * Example WaPo combined document id: `[WAPO_903cc1eab726b829294d1abdd755d5ab-1]`, or CAR: `[CAR_6869dee46ab12f0f7060874f7fc7b1c57d53144a]`
+ 
  
 ## Code and tools
 * [TREC-CAsT Tools](https://github.com/gla-ial/trec-cast-tools) repository with code and scripts for processing data.