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The SDTM allows for the representation of study days relative to the RFSTDTC reference start date variable in the DM dataset, using variables --DY, as described in Section 4.4.4, Use of the "Study Day" Variables. The calculation of additional study days within subdivisions of time in a clinical trial may be based on 1 or more sponsor-defined reference dates not represented by RFSTDTC. In such cases, the sponsor may define supplemental qualifier variables and the Define-XML document should reflect the reference dates used to calculate such study days. If the sponsor wishes to define "day within element" or "day within epoch", the reference date/time will be an element start date/time in the Subject Elements (SE) dataset (see Section 5.3, Subject Elements).


NumVariable PopulationImplementation

Study Days (--DY, --STDY, --ENDY)
  • --DY, --STDY, --ENDY represent  relative days of observations in relation to the reference date where the reference date is day 1.
  • 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 decreased 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, such as calculating duration. The raw date values should be used rather than Study Day in those calculations.

All study day values are integers. Thus, to calculate Study Day:

--DY = (date portion of --DTC) - (date portion of RFSTDTC) + 1 if --DTC is on or after RFSTDTC
--DY = (date portion of --DTC) - (date portion of RFSTDTC) if --DTC precedes RFSTDTC

This method should be used across all domains.












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