nhse faq updated

Author: Rachel Calkin

ukllc_book/docs/FAQ/pages/faq_nhse.md

@@ -1,5 +1,5 @@
 # Working with NHS England data
->Last modified: 11 Feb 2025
+>Last modified: 21 Mar 2025
 
 <details>
 <summary>Does UK LLC check the <B>accuracy</B> of health records?</summary>
@@ -21,9 +21,9 @@ More information on codes used in Electronic Health Records (EHRs) is available
 <summary>For which datasets do researchers need to provide <B>codelists</B>?</summary>
 
 Researchers must provide codelists for their projects if they intend to use any of the following datasets:
-* [HES](../../linked_health_data/NHS_England/HES%20datasets/) (Hospital Episode Statistics)
+* [HES](../../linked_health_data/NHS_England/HES%20datasets/HES_intro.md) (Hospital Episode Statistics)
 * [GDPPR](../../linked_health_data/NHS_England/Primary_care_datasets/primary_intro.md) (General Practice Extraction Service (GPES) Data for Pandemic Planning and Research)
-* [Cancer registrations](../..//linked_health_data/NHS_England/Registration%20datasets/CANCER/)
+* [Cancer registrations](../../linked_health_data/NHS_England/Registration%20datasets/CANCER/CANCER.ipynb)
 * [PCM](../../linked_health_data/NHS_England/Other%20datasets/PCM/PCM.ipynb) (Primary Care Medicines)
 
 The datasets use a range of clinical classifications, including:
@@ -39,6 +39,12 @@ More information on codes used in Electronic Health Records (EHRs) is available
 </details>
 <br>
 <details>
+<summary>How can I quantify the <b>effect of applying codelists</b> to my dataset?</summary> 
+
+The CORE file [NHSD_Presence](../../linked_health_data/NHS_England/Core%20datasets/nhsd_presence.md) contains the number of appearances and the date of the most recent appearance for each participant for each available NHS data source. Comparing LPS participants' presence in NHS data sources against the data provisioned to a project will identify which participants appear in the data source but are not included in the provisioned data. 
+</details>
+<br>
+<details>
 <summary>What impact do the <B>different levels of coding</B> have on HES data?</summary>
 
 The extent to which specific coding is used in HES data is important. For example, you may observe more records in your HESAPC (admitted patients) than in HESOP (outpatients) dataset, despite the national volume of HESOP records being typically ~5x greater per year. This is because HESAPC has meaningful diagnoses codes consistently provided, whereas generic codes are more often used in HESOP. This means when codes provided by a researcher are matched with HES data in the TRE, fewer matches ('hits') will be made on datasets with non-specific codes. Thus fewer records will be included in the project.