How are subjects selected for targeted source data verification?

If your study only requires partial source data verification (SDV), then you might opt for configuring targeted SDV and create a partial SDV strategy. This configuration defines a specific number of subjects randomly selected at a site and the specific questions that a Clinical Research Associate (CRA) must verify.

To set SDV in your study as partial targetd SDV, see Specify settings for Source Data Verification.

To create an SDV strategy, see Create a source data verification strategy and assign it to a site.

When you create a strategy for targeted SDV, you have to specify the total number of initial subjects and the percentage of remaining subjects that a CRA must verify. When the SDV strategy is assigned to a site, these numbers define the criteria for the targeted SDV subject pool, which are the selected subjects that will require verification for that site.

The Initial Subjects are selected as they are screened or added to the site (depending on the Include Screen Failures setting), until the defined number is reached. From there on, subjects are randomly selected among the Remaining Subjects to reach the defined percentage. Any subject that has not already been selected in the SDV pool and is eligible for selection, can be included in the SDV pool, even if there is a subject already selected that was added after it in the study.

Depending on whether screen failed subjects are included or not (as per the study's general SDV settings), the initial subjects may be selected differently and the calculation for the remaining subjects may be different as well:

When the Include Screen Failures setting is set to Yes, the SDV pool considers subjects for data verification as soon as they have been added to the site.
Given that screen failed subjects are eligible for SDV, the total count of subjects used to calculate the percentage of remaining subjects to be selected, also considers screen failed subjects.
When the Include Screen Failures setting is set to No, the SDV subject pool doesn't consider subjects for data verification until their status is Enrolled or Randomized. When considering the initial subjects selection, subjects are evaluated in the order that they are added to a site and a subject is not skipped for data verification unless it gets a status of Screen Failed, even if a subject added later is enrolled first.
Given that screen failed subjects are not eligible for SDV, newly added subjects, screen failed subjects, and subjects whose screening is in progress are excluded from the total count of subjects, which is used to calculate the percentage of remaining subjects to be selected. These subjects can only be later included in the total count if they become eligible for SDV, and this may happen if they later get successfully screened or become completed.

Note:
Questions that are marked as SDV for All Subjects still require verification for excluded subjects.

Let's consider the following example: A CRA must verify 3 initial subjects at a site and an additional 25% of the remaining subjects.

If screen failures are included:

After the first three added subjects are selected for SDV, the system continuously strives to match and maintain the percentage of subjects from the remaining subjects (excluding the first three) to be randomly selected, which is 25%. However, this calculation is always converted to the next whole number to ensure the defined percetage is reached. For instance, if there is only 1 remaining subject, the 25% percent of all remaining subjects would be 0.25, but since the system cannot select 0.25 subjects, 1 subject is selected (which is the next whole number). For this reason, the first subject added after the initial subjects subset will always be selected as part of the remaining subjects subset. When a new subject is added, if that subject causes the number of randomly selected subjects to fall below the specified 25% threshold, then the system randomly selects a subject among all eligible subjects that aren't already selected.

If screen failures are excluded:

In this scenario, the initial subjects are selected only once they are enrolled or randomized (depending on the study protocol). So the first 3 subjects that get added to a site, and are either enrolled or randomized, are selected. If there is any subject that fails screening, the next added subject is considered to take its place in the initial subject pool, if it gets sucessfully screened and either enrolled or randomized. So for instance, if subject 3 becomes screen failed, and subject 4 is successfully screened, subject 4 becomes the third and last initial subject selected for SDV. However, a subject will only get skipped from the initial subject count once it gets screen failed. So, following the same example, if subject 4 is randomized while subject 3 is not yet screen failed, the system reserves the spot for subject 3 as an initial subject in case it gets randomized. At this point, subject 4 is randomized, which makes it eligible for SDV as a remaining subject. At the time subject 3 is screen failed, the system makes a new calculation and identifies subject 4 as the 3rd initial subject (regardless if it was already selected for SDV as a remaining subject). Then, the system evaluates the percentage of remaining subjects that require SDV, making a new selection out of the eligible remaining subjects if necessary.

The calculation for the remaining subjects proceeds as explained above, but in this case it is always among the enrolled or randomized subjects added after the first 3 initial subjects selected. In this case starting from subject 5. While new and screen failed subjects are not eligible for SDV and have no effect on the total count, every time an added subject becomes enrolled or randomized, the total count of subjects increases and the remaining subjects percentage is recalculated. Again, if the increased amount of total subjects makes the selected subjects to fall below the specified 25% threshold, the system will randomly select the necessary subjects to meet the defined percentage.

Updates that may impact subject selection

During the study conduct period, you may update SDV settings and the SDV strategy assigned to a site. Some of these updates may impact SDV requirements for a question and subject, thus the subject selection for SDV may change.

If you update the Include Screen Failures setting from Yes to No after adding subjects in a specific study mode (Testing or Production), any screen failed subjects previously selected for the Initial Subjects or Remaining Subjects pools get excluded and no longer require data verification.

Note:
If there was any data previously verified for these subjects, that data remains verified unless it is subsequently changed. Any subsequent data changes will not impact the SDV status of the excluded subjects. For more information on changes to the SDV status, see About Source Data Verification statuses.

A new subject is to be selected only if the site's current strategy requires replacement. For example, if you had selected three initial subjects and one of them was a screen failure and gets deselected, a new subject will be selected to meet the initial subjects count. But, if you also changed the strategy to two initial subjects, no new subject would be selected since two initial subjects would remain.
If you update the Include Screen Failures setting from No to Yes, no changes occur for existing screen failed subjects, although they become eligible for SDV moving forward, based on your existing SDV strategy's algorithm.
If a screen failed subject gets withdrawn or completed, it is no longer considered as screen failed but as either withdrawn or completed. Completed subjects are eligible for selection of the SDV subject pool, but withdrawn subjects are not.

Note:
These scenarios are only possible if the Screen Failed Subjects setting in the study settings is set to Allow Withdrawal or Allow Completion respectively. See Specify study, enrollment, and visits settings.
If you increase the number of initial subjects in the SDV strategy after remaining subjects have already been selected for the SDV pool, the system selects the immediate next subjects that were added or screened for the initial subjects pool. Then, the system makes a new calculation for the remaining subjects, and randomly selects additional subjects if the current selection drops below the specified percentage.
If the SDV strategy changes to reduce the number of subjects requiring SDV, a subject already selected for SDV cannot be deselected. The new strategy will apply to newly entered subjects and the system will adjust to match and maintain the percentage of selected subjects as the total number of subjects increases.

For other impacts on your study when the SDV settings change, see How do changes in Source Data Verification settings impact a study?