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Relative timing variables are sets of variables that provide information about how Timeframes, fixed points in time, and other milestones may be defined in the context of a study as references to which the timing of the collected observation relates to either the study reference period or another fixed point in time. CDASH relative timing variables are collected for observations where a date either is not collected or is not available. The CDASH set of variables serve as an indicator (or flag) that the observation's "start" was prior to the study reference period or prior to another fixed point in time OR that the observation's "end" was after or ongoing as of the study reference period or another fixed point in time. The CDASH variables of --PRIOR and --ONGO serve this purpose. How these collected flags are translated to tabulation datasets (according to controlled terminology) depends on whether the comparison is against the protocol-defined study reference period or against another fixed point in time serving as the "reference" for the timing of the record. To emphasize, collection of these CDASH relative timing variables is always dependent on the actual date either being prospectively not collected or not available.For some studies, a timeframe (the study reference period) may be defined per protocol, company policy, standard operating procedures, or other documented procedures. The start and end dates of the study reference period are represented in tabulation variables RFSTDTC and RFENDTC.Regardless of how a study reference period is defined, the dates (and optionally times) of the start and end of that period can be collected.

If there is a need to collect information about whether an observation of interest occurred prior to a reference point or milestone other than the beginning of the study reference period or was ongoing or continuing at some reference point or milestone in the study other than the end of the defined study reference period, the date/time of that reference point or milestone should also be collected. If this date/time has been collected, reasonable comparisons can be made to that date/time with “prior”, “coincident”, “continuing”, or “ongoing” questions.

Figure 1 provides a representation of an intervention as it relates to the study reference period, and Figures 2 and 3 provide a representation of comparisons related to other fixed points in time or a milestone.

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The following steps should be taken to ensure observations of interest that occur over time can be related to the study reference period or to a fixed point in time/milestone in a meaningful way. 

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1. Define the “on-study” period (B-C). Once the overall on-study period has been defined (B-C), collect the dates/times of the start of the study reference period (e.g., date of informed consent, date of first dose) and end of the study reference period (e.g., date of last contact, date of last dose), as part of the clinical data with their respective domains (e.g., Disposition (DS), Exposure (EX)). These dates will map into the RFSTDTC (B; start of Study Reference Period) and RFENDTC (C; end of Study Reference Period) variables in the SDTMIG Demographics (DM) dataset.

2. Collected comparisons (D, E) use CDASHIG variables (e.g., “prior”, “ongoing”) for when something started or ended, in relation to the on-study reference period (i.e., RFSTDTC-RFENDTC: B-C). These CDASH variables are used to populate the SDTMIG variables--STRF and --ENRF variables when the SDTM-based datasets are created.
   Note: These relative timing variables are only populated in the SDTM -based datasets when a date is not collected.

other collected observations are related. In such cases, timeframes are referred to as "reference periods" and such fixed points in time and milestones are referred to as "reference time points." The following will be collected using TIG collection or CDASH Model timing fields when references for the timing of other observations are defined:

• Start and end dates, and times as applicable, for reference periods
• Dates, and times as applicable, for reference time points

The date, and time as applicable, of an observation to be related to a reference will be collected whenever possible. Only in cases where it is not possible to collect the date of a related observation (e.g., the date is not available), will a relative timing for the observation in relation to a reference be collected. Relative timing will be collected using collection variables:

• --PRIOR to indicate the timing of the observation was prior to the start date of a reference period or the date of a reference time point; or
• --ONGO to indicate the timing of the observation was ongoing relative to the end date of a reference period or the date of a reference time point.

The following steps should be taken when designing CRFs to ensure observations of interest can be related to reference periods and reference time points. 

1. Define the reference period and/or reference time point(s) in the context of the study. 
2. Collect the start and end date/times of the reference period and/or the date/time of the reference time point(s) using appropriate TIG collection or CDASH Model fields. 
3. Collect the actual timing of observations when it is possible to collect the date(s) for a related observation.
4. Collect relative timing of observations when collection of actual dates is not possible using fields --PRIOR and --ONGO (e.g., “prior”, “ongoing”) for when an observation started or continued, in relation to the study reference period and/or reference time point(s).

The figures below show examples of collection of relative timing in relation to a reference period and reference time points; the table that follows describes the figures. Representation of collected reference dates and relative timing in subsequent tabulation datasets is noted below and further described in Section 2.8.7.6, Representing References and Relative Timing.

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