**StudySAS Blog: Mastering Clinical Data Management with SAS**

Understanding RFXSTDTC and RFSTDTC in the Demographics (DM) Domain Understanding RFXSTDTC and RFSTDTC in the Demographics (DM) Domain Introduction In the context of clinical trials, accurately capturing key dates related to subject participation is critical for understanding the timeline of the study. The SDTM (Study Data Tabulation Model) Demographics (DM) domain includes several variables that record these key dates, two of the most important being RFXSTDTC and RFSTDTC . Although they may seem similar, these variables have distinct meanings and uses. This article explains the difference between RFXSTDTC and RFSTDTC , with detailed examples to illustrate their appropriate use. Definitions RFSTDTC (Reference Start Date/Time of Study Participation) RFSTDTC refers to the date and time when the subject officially started participating in the study. This is usually the date of randomization, the first study-sp...

USE OF THE “STUDY DAY” VARIABLES The permissible Study Day variables (--DY, --STDY, and --ENDY) describe the relative day of the observation starting with the reference date as Day 1. They are determined by comparing the date portion of the respective date/time variables (--DTC, --STDTC, and --ENDTC) to the date portion of the Subject Reference Start Date (RFSTDTC from the Demographics domain). The Subject Reference Start Date (RFSTDTC) is designated as Study Day 1. The Study Day value is incremented by 1 for each date following RFSTDTC. Dates prior to RFSTDTC are decremented by 1, with the date preceding RFSTDTC designated as Study Day -1 (there is no Study Day 0). This algorithm for determining Study Day is consistent with how people typically describe sequential days relative to a fixed reference point, but creates problems if used for mathematical calculations because it does not allow for a Day 0. As such, Study Day is not suited for use in subsequent numerical computations...