We have started implementing FHIR Questionnaire and QuestionnaireResponse for our commercial survey application (QuestLink) and intend to make that the primary API.

It is common for surveys to have a limited time during which answering the survey is possible, for example:

• User must respond to survey at least x days before a planned Encounter
  - User must respond to survey within x-y days after the related Encounter

With our user community we have defined the following way to deal with the end of period in which the survey has to be completed:
- If the reponser has provided no answers at all, the QuestionnaireResponse has to move to a final status of "expired"
- If the responder has answered 1 or more questions, but didn't complete the survey, the QuestionnaireResponse has to move to a final status of "incomplete".

The 3 statuses "complete", "incomplete" or "expired" are so-called final statuses for the QuestionnaireResponse, and only QuestionnaireResponse resources with a final status are available for further processing.

As an implementer, the only way to add statuses like "incomplete" and "expired" is through an extension.

Our preference, however, is that the valueset QuestionnaireResponseStatus is extended with codes "incomplete" and "expired", because these seems generic enough to support a wider audience.